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* [musl] [PATCH] Remove unnecessary if in __secs_to_tm
@ 2021-02-28 15:09 Mattias Andrée
  2021-02-28 17:06 ` Rich Felker
                   ` (2 more replies)
  0 siblings, 3 replies; 12+ messages in thread
From: Mattias Andrée @ 2021-02-28 15:09 UTC (permalink / raw)
  To: musl; +Cc: Mattias Andrée

Since years divisible by 100 but not by 400 are not leap years,
q_cycles can at most be 24 (DAYS_PER_100Y / DAYS_PER_4Y == 24).
---
 src/time/__secs_to_tm.c | 3 +--
 1 file changed, 1 insertion(+), 2 deletions(-)

diff --git a/src/time/__secs_to_tm.c b/src/time/__secs_to_tm.c
index 093d9021..2d0c0b2c 100644
--- a/src/time/__secs_to_tm.c
+++ b/src/time/__secs_to_tm.c
@@ -44,8 +44,7 @@ int __secs_to_tm(long long t, struct tm *tm)
 	remdays -= c_cycles * DAYS_PER_100Y;
 
 	q_cycles = remdays / DAYS_PER_4Y;
-	if (q_cycles == 25) q_cycles--;
-	remdays -= q_cycles * DAYS_PER_4Y;
+	remdays %= DAYS_PER_4Y;
 
 	remyears = remdays / 365;
 	if (remyears == 4) remyears--;
-- 
2.30.1


^ permalink raw reply	[flat|nested] 12+ messages in thread

* Re: [musl] [PATCH] Remove unnecessary if in __secs_to_tm
  2021-02-28 15:09 [musl] [PATCH] Remove unnecessary if in __secs_to_tm Mattias Andrée
@ 2021-02-28 17:06 ` Rich Felker
  2021-02-28 17:24   ` Mattias Andrée
  2021-02-28 19:22 ` [musl] [PATCH v2 1/2] " Mattias Andrée
  2021-02-28 19:27 ` [musl] [PATCH v3 1/2] Remove unnecessary if " Mattias Andrée
  2 siblings, 1 reply; 12+ messages in thread
From: Rich Felker @ 2021-02-28 17:06 UTC (permalink / raw)
  To: Mattias Andrée; +Cc: musl

On Sun, Feb 28, 2021 at 04:09:12PM +0100, Mattias Andrée wrote:
> Since years divisible by 100 but not by 400 are not leap years,
> q_cycles can at most be 24 (DAYS_PER_100Y / DAYS_PER_4Y == 24).
> ---
>  src/time/__secs_to_tm.c | 3 +--
>  1 file changed, 1 insertion(+), 2 deletions(-)
> 
> diff --git a/src/time/__secs_to_tm.c b/src/time/__secs_to_tm.c
> index 093d9021..2d0c0b2c 100644
> --- a/src/time/__secs_to_tm.c
> +++ b/src/time/__secs_to_tm.c
> @@ -44,8 +44,7 @@ int __secs_to_tm(long long t, struct tm *tm)
>  	remdays -= c_cycles * DAYS_PER_100Y;
>  
>  	q_cycles = remdays / DAYS_PER_4Y;
> -	if (q_cycles == 25) q_cycles--;
> -	remdays -= q_cycles * DAYS_PER_4Y;
> +	remdays %= DAYS_PER_4Y;
>  
>  	remyears = remdays / 365;
>  	if (remyears == 4) remyears--;

I think you're right about the condition being impossible -- it looks
like the error in thinking was that, while 400Y and 4Y are strictly
larger than 4*100Y and 4*1Y respectively, 100Y is smaller than 25*4Y.

However, changing the -= to %= is not desirable. The point of the -=
has nothing to do with the edge case that can't happen; it's to avoid
a modulo operation. Since the divisor is a constant though maybe the
compiler can generate the same code for both, anyway..?

Rich

^ permalink raw reply	[flat|nested] 12+ messages in thread

* Re: [musl] [PATCH] Remove unnecessary if in __secs_to_tm
  2021-02-28 17:06 ` Rich Felker
@ 2021-02-28 17:24   ` Mattias Andrée
  2021-02-28 17:34     ` Rich Felker
  0 siblings, 1 reply; 12+ messages in thread
From: Mattias Andrée @ 2021-02-28 17:24 UTC (permalink / raw)
  To: musl

On Sun, 28 Feb 2021 12:06:15 -0500
Rich Felker <dalias@libc.org> wrote:

> On Sun, Feb 28, 2021 at 04:09:12PM +0100, Mattias Andrée wrote:
> > Since years divisible by 100 but not by 400 are not leap years,
> > q_cycles can at most be 24 (DAYS_PER_100Y / DAYS_PER_4Y == 24).
> > ---
> >  src/time/__secs_to_tm.c | 3 +--
> >  1 file changed, 1 insertion(+), 2 deletions(-)
> > 
> > diff --git a/src/time/__secs_to_tm.c b/src/time/__secs_to_tm.c
> > index 093d9021..2d0c0b2c 100644
> > --- a/src/time/__secs_to_tm.c
> > +++ b/src/time/__secs_to_tm.c
> > @@ -44,8 +44,7 @@ int __secs_to_tm(long long t, struct tm *tm)
> >  	remdays -= c_cycles * DAYS_PER_100Y;
> >  
> >  	q_cycles = remdays / DAYS_PER_4Y;
> > -	if (q_cycles == 25) q_cycles--;
> > -	remdays -= q_cycles * DAYS_PER_4Y;
> > +	remdays %= DAYS_PER_4Y;
> >  
> >  	remyears = remdays / 365;
> >  	if (remyears == 4) remyears--;  
> 
> I think you're right about the condition being impossible -- it looks
> like the error in thinking was that, while 400Y and 4Y are strictly
> larger than 4*100Y and 4*1Y respectively, 100Y is smaller than 25*4Y.
> 
> However, changing the -= to %= is not desirable. The point of the -=
> has nothing to do with the edge case that can't happen; it's to avoid
> a modulo operation. Since the divisor is a constant though maybe the
> compiler can generate the same code for both, anyway..?
> 
> Rich

For x86_64 `remdays %= DAYS_PER_4Y` just becomes a move.

	divmod in

		int r = 52, q;
		void divmod(void)
		{
		        q = r / 111;
		        r %= 111;
		}

	becomes

		movl	r(%rip), %eax
		movl	$111, %ecx
		cltd
		idivl	%ecx
		movl	%eax, q(%rip)
		movl	%edx, r(%rip)
		ret

`remdays -= q_cycles * DAYS_PER_4Y;` on the other hand
becomes a move, a multiplication, and an addition.

	divmod in

		int r = 52, q;
		void divmod(void)
		{
		        q = r / 111;
		        r -= q * 111;
		}

	becomes

		movl	r(%rip), %eax
		movl	$111, %ecx
		cltd
		idivl	%ecx
		movl	%eax, q(%rip)
		imull	$-111, %eax, %eax
		addl	r(%rip), %eax
		movl	%eax, r(%rip)
		ret

So I would say %= is the better option, at least for x86_64.

Of course, if you prefer, I will change it to use -=.

^ permalink raw reply	[flat|nested] 12+ messages in thread

* Re: [musl] [PATCH] Remove unnecessary if in __secs_to_tm
  2021-02-28 17:24   ` Mattias Andrée
@ 2021-02-28 17:34     ` Rich Felker
  0 siblings, 0 replies; 12+ messages in thread
From: Rich Felker @ 2021-02-28 17:34 UTC (permalink / raw)
  To: Mattias Andrée; +Cc: musl

On Sun, Feb 28, 2021 at 06:24:45PM +0100, Mattias Andrée wrote:
> On Sun, 28 Feb 2021 12:06:15 -0500
> Rich Felker <dalias@libc.org> wrote:
> 
> > On Sun, Feb 28, 2021 at 04:09:12PM +0100, Mattias Andrée wrote:
> > > Since years divisible by 100 but not by 400 are not leap years,
> > > q_cycles can at most be 24 (DAYS_PER_100Y / DAYS_PER_4Y == 24).
> > > ---
> > >  src/time/__secs_to_tm.c | 3 +--
> > >  1 file changed, 1 insertion(+), 2 deletions(-)
> > > 
> > > diff --git a/src/time/__secs_to_tm.c b/src/time/__secs_to_tm.c
> > > index 093d9021..2d0c0b2c 100644
> > > --- a/src/time/__secs_to_tm.c
> > > +++ b/src/time/__secs_to_tm.c
> > > @@ -44,8 +44,7 @@ int __secs_to_tm(long long t, struct tm *tm)
> > >  	remdays -= c_cycles * DAYS_PER_100Y;
> > >  
> > >  	q_cycles = remdays / DAYS_PER_4Y;
> > > -	if (q_cycles == 25) q_cycles--;
> > > -	remdays -= q_cycles * DAYS_PER_4Y;
> > > +	remdays %= DAYS_PER_4Y;
> > >  
> > >  	remyears = remdays / 365;
> > >  	if (remyears == 4) remyears--;  
> > 
> > I think you're right about the condition being impossible -- it looks
> > like the error in thinking was that, while 400Y and 4Y are strictly
> > larger than 4*100Y and 4*1Y respectively, 100Y is smaller than 25*4Y.
> > 
> > However, changing the -= to %= is not desirable. The point of the -=
> > has nothing to do with the edge case that can't happen; it's to avoid
> > a modulo operation. Since the divisor is a constant though maybe the
> > compiler can generate the same code for both, anyway..?
> > 
> > Rich
> 
> For x86_64 `remdays %= DAYS_PER_4Y` just becomes a move.
> 
> 	divmod in
> 
> 		int r = 52, q;
> 		void divmod(void)
> 		{
> 		        q = r / 111;
> 		        r %= 111;
> 		}
> 
> 	becomes
> 
> 		movl	r(%rip), %eax
> 		movl	$111, %ecx
> 		cltd
> 		idivl	%ecx
> 		movl	%eax, q(%rip)
> 		movl	%edx, r(%rip)
> 		ret
> 
> `remdays -= q_cycles * DAYS_PER_4Y;` on the other hand
> becomes a move, a multiplication, and an addition.
> 
> 	divmod in
> 
> 		int r = 52, q;
> 		void divmod(void)
> 		{
> 		        q = r / 111;
> 		        r -= q * 111;
> 		}
> 
> 	becomes
> 
> 		movl	r(%rip), %eax
> 		movl	$111, %ecx
> 		cltd
> 		idivl	%ecx
> 		movl	%eax, q(%rip)
> 		imull	$-111, %eax, %eax
> 		addl	r(%rip), %eax
> 		movl	%eax, r(%rip)
> 		ret
> 
> So I would say %= is the better option, at least for x86_64.
> 
> Of course, if you prefer, I will change it to use -=.

It's an unrelated change, so if it should be made it should be done as
a different commit, and in all the places not just arbitrarily in one
of them. But the above analysis is probably not indicative. You're
dividing by a variable, in which case on x86_64 idiv gets used and
there's a remainder available for free. But in the code here all the
divisions are by constants and should cause the compiler to emit code
using only multiplies.

(Note: this may not currently be the case with -Os, which is one big
reason we should be dropping -Os and instead tuning -O2 to behave
better, which is a longstanding agenda item).

Also, if these were actual div/mod operations by a variable, the
interesting case is not archs with an instruction that produces the
remainder for free, but ones where two separate operations are
required or where long division in software is required.

Rich

^ permalink raw reply	[flat|nested] 12+ messages in thread

* [musl] [PATCH v2 1/2] Remove unnecessary if in __secs_to_tm
  2021-02-28 15:09 [musl] [PATCH] Remove unnecessary if in __secs_to_tm Mattias Andrée
  2021-02-28 17:06 ` Rich Felker
@ 2021-02-28 19:22 ` Mattias Andrée
  2021-02-28 19:22   ` [musl] [PATCH v2 2/2] Use modulo instead of mul+sub " Mattias Andrée
  2021-02-28 19:27 ` [musl] [PATCH v3 1/2] Remove unnecessary if " Mattias Andrée
  2 siblings, 1 reply; 12+ messages in thread
From: Mattias Andrée @ 2021-02-28 19:22 UTC (permalink / raw)
  To: musl; +Cc: Mattias Andrée

Since years divisible by 100 but not by 400 are not leap years,
q_cycles can at most be 24 (DAYS_PER_100Y / DAYS_PER_4Y == 24).
---
 src/time/__secs_to_tm.c | 1 -
 1 file changed, 1 deletion(-)

diff --git a/src/time/__secs_to_tm.c b/src/time/__secs_to_tm.c
index 093d9021..62219df5 100644
--- a/src/time/__secs_to_tm.c
+++ b/src/time/__secs_to_tm.c
@@ -44,7 +44,6 @@ int __secs_to_tm(long long t, struct tm *tm)
 	remdays -= c_cycles * DAYS_PER_100Y;
 
 	q_cycles = remdays / DAYS_PER_4Y;
-	if (q_cycles == 25) q_cycles--;
 	remdays -= q_cycles * DAYS_PER_4Y;
 
 	remyears = remdays / 365;
-- 
2.30.1


^ permalink raw reply	[flat|nested] 12+ messages in thread

* [musl] [PATCH v2 2/2] Use modulo instead of mul+sub in __secs_to_tm
  2021-02-28 19:22 ` [musl] [PATCH v2 1/2] " Mattias Andrée
@ 2021-02-28 19:22   ` Mattias Andrée
  2021-02-28 19:37     ` Szabolcs Nagy
  0 siblings, 1 reply; 12+ messages in thread
From: Mattias Andrée @ 2021-02-28 19:22 UTC (permalink / raw)
  To: musl; +Cc: Mattias Andrée

On x86 modulo is free when doing division, so this removes
a multiplication and at the cost of replacing a conditional
move with a conditional jump, but it still appears to be
faster.
(Similar architectures: nds32le)

ARM doesn't have modulo, instead an multiply-and-subtract
operation is done after the division, so the diffence
here is either none at all, or a move and a multiply-and-add
being replaced with a multiply-and-subtract.
(Similar architectures: or1k)

RISC-V on the other hand has a separate modulo
instruction and will perform a separate modulo instead of
an assignment, a multiplication, and an addition with
this change. GCC does change how the modulo operation is
realised depending on the optimisation level. I don't know
how this affects the performance, however a simple test on
x86 suggests that doing a modulo operations is actually
faster than assign–multiply–add.
---
 src/time/__secs_to_tm.c | 18 +++++++++++++++---
 1 file changed, 15 insertions(+), 3 deletions(-)

diff --git a/src/time/__secs_to_tm.c b/src/time/__secs_to_tm.c
index 62219df5..348e51ec 100644
--- a/src/time/__secs_to_tm.c
+++ b/src/time/__secs_to_tm.c
@@ -39,16 +39,28 @@ int __secs_to_tm(long long t, struct tm *tm)
 		qc_cycles--;
 	}
 
+#if 1
+	c_cycles = remdays / DAYS_PER_100Y;
+	remdays %= DAYS_PER_100Y;
+	if (c_cycles == 4) {
+		remdays += DAYS_PER_100Y;
+		c_cycles--;
+	}
+#else
 	c_cycles = remdays / DAYS_PER_100Y;
 	if (c_cycles == 4) c_cycles--;
 	remdays -= c_cycles * DAYS_PER_100Y;
+#endif
 
 	q_cycles = remdays / DAYS_PER_4Y;
-	remdays -= q_cycles * DAYS_PER_4Y;
+	remdays %= DAYS_PER_4Y;
 
 	remyears = remdays / 365;
-	if (remyears == 4) remyears--;
-	remdays -= remyears * 365;
+	remdays %= 365;
+	if (remyears == 4) {
+		remdays += 365;
+		remyears--;
+	}
 
 	leap = !remyears && (q_cycles || !c_cycles);
 	yday = remdays + 31 + 28 + leap;
-- 
2.30.1


^ permalink raw reply	[flat|nested] 12+ messages in thread

* [musl] [PATCH v3 1/2] Remove unnecessary if in __secs_to_tm
  2021-02-28 15:09 [musl] [PATCH] Remove unnecessary if in __secs_to_tm Mattias Andrée
  2021-02-28 17:06 ` Rich Felker
  2021-02-28 19:22 ` [musl] [PATCH v2 1/2] " Mattias Andrée
@ 2021-02-28 19:27 ` Mattias Andrée
  2021-02-28 19:27   ` [musl] [PATCH v3 2/2] Use modulo instead of mul+sub " Mattias Andrée
  2 siblings, 1 reply; 12+ messages in thread
From: Mattias Andrée @ 2021-02-28 19:27 UTC (permalink / raw)
  To: musl; +Cc: Mattias Andrée

Since years divisible by 100 but not by 400 are not leap years,
q_cycles can at most be 24 (DAYS_PER_100Y / DAYS_PER_4Y == 24).
---
 src/time/__secs_to_tm.c | 1 -
 1 file changed, 1 deletion(-)

diff --git a/src/time/__secs_to_tm.c b/src/time/__secs_to_tm.c
index 093d9021..62219df5 100644
--- a/src/time/__secs_to_tm.c
+++ b/src/time/__secs_to_tm.c
@@ -44,7 +44,6 @@ int __secs_to_tm(long long t, struct tm *tm)
 	remdays -= c_cycles * DAYS_PER_100Y;
 
 	q_cycles = remdays / DAYS_PER_4Y;
-	if (q_cycles == 25) q_cycles--;
 	remdays -= q_cycles * DAYS_PER_4Y;
 
 	remyears = remdays / 365;
-- 
2.30.1


^ permalink raw reply	[flat|nested] 12+ messages in thread

* [musl] [PATCH v3 2/2] Use modulo instead of mul+sub in __secs_to_tm
  2021-02-28 19:27 ` [musl] [PATCH v3 1/2] Remove unnecessary if " Mattias Andrée
@ 2021-02-28 19:27   ` Mattias Andrée
  0 siblings, 0 replies; 12+ messages in thread
From: Mattias Andrée @ 2021-02-28 19:27 UTC (permalink / raw)
  To: musl; +Cc: Mattias Andrée

On x86 modulo is free when doing division, so this removes
a multiplication and at the cost of replacing a conditional
move with a conditional jump, but it still appears to be
faster.
(Similar architectures: nds32le)

ARM doesn't have modulo, instead an multiply-and-subtract
operation is done after the division, so the diffence
here is either none at all, or a move and a multiply-and-add
being replaced with a multiply-and-subtract.
(Similar architectures: or1k)

RISC-V on the other hand has a separate modulo
instruction and will perform a separate modulo instead of
an assignment, a multiplication, and an addition with
this change. GCC does change how the modulo operation is
realised depending on the optimisation level. I don't know
how this affects the performance, however a simple test on
x86 suggests that doing a modulo operations is actually
faster than assign–multiply–add.
---
 src/time/__secs_to_tm.c | 16 +++++++++++-----
 1 file changed, 11 insertions(+), 5 deletions(-)

diff --git a/src/time/__secs_to_tm.c b/src/time/__secs_to_tm.c
index 62219df5..59b1fc8d 100644
--- a/src/time/__secs_to_tm.c
+++ b/src/time/__secs_to_tm.c
@@ -40,15 +40,21 @@ int __secs_to_tm(long long t, struct tm *tm)
 	}
 
 	c_cycles = remdays / DAYS_PER_100Y;
-	if (c_cycles == 4) c_cycles--;
-	remdays -= c_cycles * DAYS_PER_100Y;
+	remdays %= DAYS_PER_100Y;
+	if (c_cycles == 4) {
+		remdays += DAYS_PER_100Y;
+		c_cycles--;
+	}
 
 	q_cycles = remdays / DAYS_PER_4Y;
-	remdays -= q_cycles * DAYS_PER_4Y;
+	remdays %= DAYS_PER_4Y;
 
 	remyears = remdays / 365;
-	if (remyears == 4) remyears--;
-	remdays -= remyears * 365;
+	remdays %= 365;
+	if (remyears == 4) {
+		remdays += 365;
+		remyears--;
+	}
 
 	leap = !remyears && (q_cycles || !c_cycles);
 	yday = remdays + 31 + 28 + leap;
-- 
2.30.1


^ permalink raw reply	[flat|nested] 12+ messages in thread

* Re: [musl] [PATCH v2 2/2] Use modulo instead of mul+sub in __secs_to_tm
  2021-02-28 19:22   ` [musl] [PATCH v2 2/2] Use modulo instead of mul+sub " Mattias Andrée
@ 2021-02-28 19:37     ` Szabolcs Nagy
  2021-02-28 19:52       ` Mattias Andrée
  2021-02-28 19:58       ` Jon Chesterfield
  0 siblings, 2 replies; 12+ messages in thread
From: Szabolcs Nagy @ 2021-02-28 19:37 UTC (permalink / raw)
  To: Mattias Andrée; +Cc: musl

* Mattias Andrée <maandree@kth.se> [2021-02-28 20:22:10 +0100]:
> On x86 modulo is free when doing division, so this removes

there should be no division.

div by const is transformed to mul and shift at -O1 and
that's what we should be using instead of manual hacks.

https://godbolt.org/z/Wsxq5h

> a multiplication and at the cost of replacing a conditional
> move with a conditional jump, but it still appears to be
> faster.
> (Similar architectures: nds32le)
> 
> ARM doesn't have modulo, instead an multiply-and-subtract
> operation is done after the division, so the diffence
> here is either none at all, or a move and a multiply-and-add
> being replaced with a multiply-and-subtract.
> (Similar architectures: or1k)
> 
> RISC-V on the other hand has a separate modulo
> instruction and will perform a separate modulo instead of
> an assignment, a multiplication, and an addition with
> this change. GCC does change how the modulo operation is
> realised depending on the optimisation level. I don't know
> how this affects the performance, however a simple test on
> x86 suggests that doing a modulo operations is actually
> faster than assign–multiply–add.

did you benchmark with CFLAGS=-O2 or -Os ?

> ---
>  src/time/__secs_to_tm.c | 18 +++++++++++++++---
>  1 file changed, 15 insertions(+), 3 deletions(-)
> 
> diff --git a/src/time/__secs_to_tm.c b/src/time/__secs_to_tm.c
> index 62219df5..348e51ec 100644
> --- a/src/time/__secs_to_tm.c
> +++ b/src/time/__secs_to_tm.c
> @@ -39,16 +39,28 @@ int __secs_to_tm(long long t, struct tm *tm)
>  		qc_cycles--;
>  	}
>  
> +#if 1
> +	c_cycles = remdays / DAYS_PER_100Y;
> +	remdays %= DAYS_PER_100Y;
> +	if (c_cycles == 4) {
> +		remdays += DAYS_PER_100Y;
> +		c_cycles--;
> +	}
> +#else
>  	c_cycles = remdays / DAYS_PER_100Y;
>  	if (c_cycles == 4) c_cycles--;
>  	remdays -= c_cycles * DAYS_PER_100Y;
> +#endif
>  
>  	q_cycles = remdays / DAYS_PER_4Y;
> -	remdays -= q_cycles * DAYS_PER_4Y;
> +	remdays %= DAYS_PER_4Y;
>  
>  	remyears = remdays / 365;
> -	if (remyears == 4) remyears--;
> -	remdays -= remyears * 365;
> +	remdays %= 365;
> +	if (remyears == 4) {
> +		remdays += 365;
> +		remyears--;
> +	}
>  
>  	leap = !remyears && (q_cycles || !c_cycles);
>  	yday = remdays + 31 + 28 + leap;
> -- 
> 2.30.1

^ permalink raw reply	[flat|nested] 12+ messages in thread

* Re: [musl] [PATCH v2 2/2] Use modulo instead of mul+sub in __secs_to_tm
  2021-02-28 19:37     ` Szabolcs Nagy
@ 2021-02-28 19:52       ` Mattias Andrée
  2021-02-28 19:58       ` Jon Chesterfield
  1 sibling, 0 replies; 12+ messages in thread
From: Mattias Andrée @ 2021-02-28 19:52 UTC (permalink / raw)
  To: musl

On Sun, 28 Feb 2021 20:37:33 +0100
Szabolcs Nagy <nsz@port70.net> wrote:

> * Mattias Andrée <maandree@kth.se> [2021-02-28 20:22:10 +0100]:
> > On x86 modulo is free when doing division, so this removes  
> 
> there should be no division.
> 
> div by const is transformed to mul and shift at -O1 and
> that's what we should be using instead of manual hacks.
> 
> https://godbolt.org/z/Wsxq5h

For -Os, the currently used optimisation, it does division.
But for other optimisations, it makes no difference as the
compiler will do a multiply–subtract either way.

> 
> > a multiplication and at the cost of replacing a conditional
> > move with a conditional jump, but it still appears to be
> > faster.
> > (Similar architectures: nds32le)
> > 
> > ARM doesn't have modulo, instead an multiply-and-subtract
> > operation is done after the division, so the diffence
> > here is either none at all, or a move and a multiply-and-add
> > being replaced with a multiply-and-subtract.
> > (Similar architectures: or1k)
> > 
> > RISC-V on the other hand has a separate modulo
> > instruction and will perform a separate modulo instead of
> > an assignment, a multiplication, and an addition with
> > this change. GCC does change how the modulo operation is
> > realised depending on the optimisation level. I don't know
> > how this affects the performance, however a simple test on
> > x86 suggests that doing a modulo operations is actually
> > faster than assign–multiply–add.  
> 
> did you benchmark with CFLAGS=-O2 or -Os ?

I guess it must have been -O0 or -Os, but what I did was
I made a trivial program and checked that assembly output,
to see which method was faster. The important part here
was that the compiler didn't change the division operation,
so adding optimisation might have bad the test pointless.
As I wrote, for RISC-V the compiler did exactly what was
written, no matter the optimisation level, that is, for
RISC-V I tried, -O0, -O1, -O2, -O3, and -Os.

> 
> > ---
> >  src/time/__secs_to_tm.c | 18 +++++++++++++++---
> >  1 file changed, 15 insertions(+), 3 deletions(-)
> > 
> > diff --git a/src/time/__secs_to_tm.c b/src/time/__secs_to_tm.c
> > index 62219df5..348e51ec 100644
> > --- a/src/time/__secs_to_tm.c
> > +++ b/src/time/__secs_to_tm.c
> > @@ -39,16 +39,28 @@ int __secs_to_tm(long long t, struct tm *tm)
> >  		qc_cycles--;
> >  	}
> >  
> > +#if 1
> > +	c_cycles = remdays / DAYS_PER_100Y;
> > +	remdays %= DAYS_PER_100Y;
> > +	if (c_cycles == 4) {
> > +		remdays += DAYS_PER_100Y;
> > +		c_cycles--;
> > +	}
> > +#else
> >  	c_cycles = remdays / DAYS_PER_100Y;
> >  	if (c_cycles == 4) c_cycles--;
> >  	remdays -= c_cycles * DAYS_PER_100Y;
> > +#endif
> >  
> >  	q_cycles = remdays / DAYS_PER_4Y;
> > -	remdays -= q_cycles * DAYS_PER_4Y;
> > +	remdays %= DAYS_PER_4Y;
> >  
> >  	remyears = remdays / 365;
> > -	if (remyears == 4) remyears--;
> > -	remdays -= remyears * 365;
> > +	remdays %= 365;
> > +	if (remyears == 4) {
> > +		remdays += 365;
> > +		remyears--;
> > +	}
> >  
> >  	leap = !remyears && (q_cycles || !c_cycles);
> >  	yday = remdays + 31 + 28 + leap;
> > -- 
> > 2.30.1  


^ permalink raw reply	[flat|nested] 12+ messages in thread

* Re: [musl] [PATCH v2 2/2] Use modulo instead of mul+sub in __secs_to_tm
  2021-02-28 19:37     ` Szabolcs Nagy
  2021-02-28 19:52       ` Mattias Andrée
@ 2021-02-28 19:58       ` Jon Chesterfield
  2021-03-01 19:26         ` Markus Wichmann
  1 sibling, 1 reply; 12+ messages in thread
From: Jon Chesterfield @ 2021-02-28 19:58 UTC (permalink / raw)
  To: musl, Mattias Andrée

[-- Attachment #1: Type: text/plain, Size: 800 bytes --]

On Sun, 28 Feb 2021, 19:37 Szabolcs Nagy, <nsz@port70.net> wrote:

> * Mattias Andrée <maandree@kth.se> [2021-02-28 20:22:10 +0100]:
> > On x86 modulo is free when doing division, so this removes
>
> there should be no division.
>
> div by const is transformed to mul and shift at -O1 and
> that's what we should be using instead of manual hacks.


Right. Divide by constant is cheap because compilers have a bunch of
transforms to get rid of the divide in favour of one of more cheaper
instructions.

Note that module coming for free with division doesn't make it cheap.
Integer division is far more expensive that integer multiply on ~ every
architecture. Several architectures implement division in software. It's
not cheap on x86, despite the dedicated instruction.

Cheers

[-- Attachment #2: Type: text/html, Size: 1315 bytes --]

^ permalink raw reply	[flat|nested] 12+ messages in thread

* Re: [musl] [PATCH v2 2/2] Use modulo instead of mul+sub in __secs_to_tm
  2021-02-28 19:58       ` Jon Chesterfield
@ 2021-03-01 19:26         ` Markus Wichmann
  0 siblings, 0 replies; 12+ messages in thread
From: Markus Wichmann @ 2021-03-01 19:26 UTC (permalink / raw)
  To: musl

On Sun, Feb 28, 2021 at 07:58:27PM +0000, Jon Chesterfield wrote:
> Note that module coming for free with division doesn't make it cheap.
> Integer division is far more expensive that integer multiply on ~ every
> architecture. Several architectures implement division in software. It's
> not cheap on x86, despite the dedicated instruction.
>
> Cheers

And then there's PowerPC, which does have a divide instruction but no
modulo. The manual explicitly states that if you need a modulo, you are
supposed to divide, multiply, and subtract.

Ciao,
Markus

^ permalink raw reply	[flat|nested] 12+ messages in thread

end of thread, other threads:[~2021-03-01 19:27 UTC | newest]

Thread overview: 12+ messages (download: mbox.gz / follow: Atom feed)
-- links below jump to the message on this page --
2021-02-28 15:09 [musl] [PATCH] Remove unnecessary if in __secs_to_tm Mattias Andrée
2021-02-28 17:06 ` Rich Felker
2021-02-28 17:24   ` Mattias Andrée
2021-02-28 17:34     ` Rich Felker
2021-02-28 19:22 ` [musl] [PATCH v2 1/2] " Mattias Andrée
2021-02-28 19:22   ` [musl] [PATCH v2 2/2] Use modulo instead of mul+sub " Mattias Andrée
2021-02-28 19:37     ` Szabolcs Nagy
2021-02-28 19:52       ` Mattias Andrée
2021-02-28 19:58       ` Jon Chesterfield
2021-03-01 19:26         ` Markus Wichmann
2021-02-28 19:27 ` [musl] [PATCH v3 1/2] Remove unnecessary if " Mattias Andrée
2021-02-28 19:27   ` [musl] [PATCH v3 2/2] Use modulo instead of mul+sub " Mattias Andrée

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