* [TUHS] OT: compiler back-end bug
@ 2019-09-29 9:21 Warren Toomey
2019-09-29 9:47 ` Ralph Corderoy
0 siblings, 1 reply; 6+ messages in thread
From: Warren Toomey @ 2019-09-29 9:21 UTC (permalink / raw)
To: tuhs
All, very off-topic for TUHS but you have a bounty of experience. If any
of you have Intel ia64 skills and/or fixing compiler back-end bugs, could
you contact me off-list? I'm writing a back-end for the SubC compiler and
I have 'one last bug'™ before it can compile itself, and I'm stuck.
Details at: https://minnie.tuhs.org/wktcloud/index.php/s/QdKZAqcBytoFBkQ/download?path=%2F&files=help.txt
Thanks, Warren
^ permalink raw reply [flat|nested] 6+ messages in thread
* Re: [TUHS] OT: compiler back-end bug
2019-09-29 9:21 [TUHS] OT: compiler back-end bug Warren Toomey
@ 2019-09-29 9:47 ` Ralph Corderoy
2019-09-29 10:03 ` Warren Toomey
0 siblings, 1 reply; 6+ messages in thread
From: Ralph Corderoy @ 2019-09-29 9:47 UTC (permalink / raw)
To: tuhs
Hi Warren,
> If any of you have Intel ia64 skills and/or fixing compiler back-end
> bugs, could you contact me off-list?
Sorry, on list, but it may help the willing eyeballs if
> To see the original compiler's assembly version of fwrite():
>
> $ ./scc0 -S lib/fwrite.c
>
> To see my compiler's [faulty] assembly version of fwrite():
>
> $ ./myscc -S lib/fwrite.c
these two were made easily available, e.g. a pastebin like
curl -sSF 'f:1=<-' ix.io <lib/fwrite.c
as then spotting the cause of
> At present, my compiler fwrite() code is passing bogus arguments to
> memcpy() where it crashes with a segfault.
wouldn't involve downloads, make, etc., that make it easy to think one
doesn't have the time to look. :-)
--
Cheers, Ralph.
^ permalink raw reply [flat|nested] 6+ messages in thread
* Re: [TUHS] OT: compiler back-end bug
2019-09-29 9:47 ` Ralph Corderoy
@ 2019-09-29 10:03 ` Warren Toomey
2019-09-29 10:50 ` Ralph Corderoy
0 siblings, 1 reply; 6+ messages in thread
From: Warren Toomey @ 2019-09-29 10:03 UTC (permalink / raw)
To: Ralph Corderoy; +Cc: tuhs
On Sun, Sep 29, 2019 at 10:47:26AM +0100, Ralph Corderoy wrote:
> these two were made easily available, e.g. a pastebin like
> wouldn't involve downloads, make, etc., that make it easy to think one
> doesn't have the time to look. :-)
Good point Ralph:
https://minnie.tuhs.org/wktcloud/index.php/s/HQjsggHb4i6wdWM?path=%2FSfiles
Thanks! Warren
^ permalink raw reply [flat|nested] 6+ messages in thread
* Re: [TUHS] OT: compiler back-end bug
2019-09-29 10:03 ` Warren Toomey
@ 2019-09-29 10:50 ` Ralph Corderoy
2019-10-03 20:03 ` Steve Johnson
0 siblings, 1 reply; 6+ messages in thread
From: Ralph Corderoy @ 2019-09-29 10:50 UTC (permalink / raw)
To: tuhs
Hi Warren,
> Good point Ralph:
> https://minnie.tuhs.org/wktcloud/index.php/s/HQjsggHb4i6wdWM?path=%2FSfiles
I've always tried to avoid x86 and friends for ARM, so I may be wrong,
but the run up to the first of the two memcpy() calls looks the same to
me. Here's the assembler, values given an RBP of 100, and the stack
contents. Good version first, bad second.
rbp = 100
L29:
movq -8(%rbp),%rax rax = *92
pushq %rax *92
movq 16(%rbp),%rax rax = *116
pushq %rax *92 *116
movq $64,%rax rax = 64
pushq %rax *92 *116 64
movq 32(%rbp),%rax rax = *132
popq %rcx rcx = 64 *92 *116
addq %rcx,%rax rcx = 64+*132
movq (%rax),%rax rax = *(64+*132)
pushq %rax *92 *116 *(64+*132)
movq $40,%rax rax = 40
pushq %rax *92 *116 *(64+*132) 40
movq 32(%rbp),%rax rax = *132
popq %rcx rcx = 40 *92 *116 *(64+*132)
addq %rcx,%rax rax = 40+*132
movq (%rax),%rax rax = *(40+*132)
popq %rcx rcx = *(64+*132) *92 *116
addq %rcx,%rax rax = *(64+*132)+*(40+*132)
pushq %rax *92 *116 *(64+*132)+*(40+*132)
call Cmemcpy
rbp = 100
L29:
movq -8(%rbp),%r8 r8 = *92
pushq %r8 *92
movq 16(%rbp),%r8 r8 = *116
pushq %r8 *92 *116
movq $64,%r8 r8 = 64
movq 32(%rbp),%r9 r9 = *132
addq %r9,%r8 r8 = *132+64
movq (%r8),%r8 r8 = *(*132+64)
movq $40,%r9 r9 = 40
movq 32(%rbp),%r10 r10 = *132
addq %r10,%r9 r9 = *132+40
movq (%r9),%r9 r9 = *(*132+40)
addq %r9,%r8 r8 = *(*132+64)+*(*132+40)
pushq %r8 *92 *116 *(*132+64)+*(*132+40)
call Cmemcpy
A glance at the second memcpy() call look equivalent too.
So perhaps it's not calculating the parameters to memcpy() that's wrong,
but the inputs into those calculations being faulty? I'd use gdb(1) to
break at particular instructions, examine memory, etc., to work
backwards through the bad version until spotting where good data becomes
bad.
--
Cheers, Ralph.
^ permalink raw reply [flat|nested] 6+ messages in thread
* Re: [TUHS] OT: compiler back-end bug
2019-10-03 20:03 ` Steve Johnson
@ 2019-10-03 20:03 ` Nemo Nusquam
0 siblings, 0 replies; 6+ messages in thread
From: Nemo Nusquam @ 2019-10-03 20:03 UTC (permalink / raw)
To: tuhs
On 10/03/19 16:03, Steve Johnson wrote (in part):
> Once you know what you want, then you figure out the first instruction
> that isn't right and hit it with everything you have...
+42
N.
(And sorry, Warren, I haven't written iapx assembler in over a decade.)
^ permalink raw reply [flat|nested] 6+ messages in thread
* Re: [TUHS] OT: compiler back-end bug
2019-09-29 10:50 ` Ralph Corderoy
@ 2019-10-03 20:03 ` Steve Johnson
2019-10-03 20:03 ` Nemo Nusquam
0 siblings, 1 reply; 6+ messages in thread
From: Steve Johnson @ 2019-10-03 20:03 UTC (permalink / raw)
To: Ralph Corderoy, tuhs
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I have all to much experience with back end bugs, usually when someone
porting PCC asked for help.
The advice I always gave first was: "what would the correct output
look like?"
90% of the time, they didn't know. And it's hard to hit the target
if you don't know where it is...
Once you know what you want, then you figure out the first instruction
that isn't right and hit it with everything you have...
Hope this helps...
Steve
----- Original Message -----
From: "Ralph Corderoy" <ralph@inputplus.co.uk>
To:<tuhs@tuhs.org>
Cc:
Sent:Sun, 29 Sep 2019 11:50:16 +0100
Subject:Re: [TUHS] OT: compiler back-end bug
Hi Warren,
> Good point Ralph:
>
https://minnie.tuhs.org/wktcloud/index.php/s/HQjsggHb4i6wdWM?path=%2FSfiles
I've always tried to avoid x86 and friends for ARM, so I may be
wrong,
but the run up to the first of the two memcpy() calls looks the same
to
me. Here's the assembler, values given an RBP of 100, and the stack
contents. Good version first, bad second.
rbp = 100
L29:
movq -8(%rbp),%rax rax = *92
pushq %rax *92
movq 16(%rbp),%rax rax = *116
pushq %rax *92 *116
movq $64,%rax rax = 64
pushq %rax *92 *116 64
movq 32(%rbp),%rax rax = *132
popq %rcx rcx = 64 *92 *116
addq %rcx,%rax rcx = 64+*132
movq (%rax),%rax rax = *(64+*132)
pushq %rax *92 *116 *(64+*132)
movq $40,%rax rax = 40
pushq %rax *92 *116 *(64+*132) 40
movq 32(%rbp),%rax rax = *132
popq %rcx rcx = 40 *92 *116 *(64+*132)
addq %rcx,%rax rax = 40+*132
movq (%rax),%rax rax = *(40+*132)
popq %rcx rcx = *(64+*132) *92 *116
addq %rcx,%rax rax = *(64+*132)+*(40+*132)
pushq %rax *92 *116 *(64+*132)+*(40+*132)
call Cmemcpy
rbp = 100
L29:
movq -8(%rbp),%r8 r8 = *92
pushq %r8 *92
movq 16(%rbp),%r8 r8 = *116
pushq %r8 *92 *116
movq $64,%r8 r8 = 64
movq 32(%rbp),%r9 r9 = *132
addq %r9,%r8 r8 = *132+64
movq (%r8),%r8 r8 = *(*132+64)
movq $40,%r9 r9 = 40
movq 32(%rbp),%r10 r10 = *132
addq %r10,%r9 r9 = *132+40
movq (%r9),%r9 r9 = *(*132+40)
addq %r9,%r8 r8 = *(*132+64)+*(*132+40)
pushq %r8 *92 *116 *(*132+64)+*(*132+40)
call Cmemcpy
A glance at the second memcpy() call look equivalent too.
So perhaps it's not calculating the parameters to memcpy() that's
wrong,
but the inputs into those calculations being faulty? I'd use gdb(1)
to
break at particular instructions, examine memory, etc., to work
backwards through the bad version until spotting where good data
becomes
bad.
--
Cheers, Ralph.
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^ permalink raw reply [flat|nested] 6+ messages in thread
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2019-09-29 9:21 [TUHS] OT: compiler back-end bug Warren Toomey
2019-09-29 9:47 ` Ralph Corderoy
2019-09-29 10:03 ` Warren Toomey
2019-09-29 10:50 ` Ralph Corderoy
2019-10-03 20:03 ` Steve Johnson
2019-10-03 20:03 ` Nemo Nusquam
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