Re: [musl] Draft riscv64 TLSDESC implementation

[Fangrui Song <i@maskray.me>]

On Sun, Jan 21, 2024 at 7:41 PM Tatsuyuki Ishi <ishitatsuyuki@gmail.com> wrote:
>
> > On Jan 22, 2024, at 9:03, Rich Felker <dalias@libc.org> wrote:
> >
> > On Sun, Jan 21, 2024 at 03:48:55PM -0800, Fangrui Song wrote:
> >> On Sun, Jan 21, 2024 at 2:28 PM Rich Felker <dalias@libc.org> wrote:
> >>>
> >>> On Tue, Aug 22, 2023 at 01:38:21PM -0400, Rich Felker wrote:
> >>>> The psABI work is not finalized, but based on the current status of
> >>>> https://github.com/riscv-non-isa/riscv-elf-psabi-doc/pull/373, I think
> >>>> the attached is a valid (but untested) implementation of TLSDESC for
> >>>> riscv64. Actually activating it requires also adding the relocation
> >>>> type macro to riscv64/reloc.h.
> >>>>
> >>>> If any rv folks could look it over and make sure I haven't made any
> >>>> stupid asm errors or missed any obvious optimizations, that would help
> >>>> to quickly get this merged when the psABI is finalized.
> >>>>
> >>>> Rich
> >>>
> >>>> .text
> >>>> .global __tlsdesc_static
> >>>> .hidden __tlsdesc_static
> >>>> .type __tlsdesc_static,%function
> >>>> __tlsdesc_static:
> >>>>      ld a0,8(a0)
> >>>>      jr t0
> >>>>
> >>>> .global __tlsdesc_dynamic
> >>>> .hidden __tlsdesc_dynamic
> >>>> .type __tlsdesc_dynamic,%function
> >>>> __tlsdesc_dynamic:
> >>>>      add sp,sp,-8
> >>>>      sd t1,(sp)
> >>>>      sd t2,8(sp)
> >>>>
> >>>>      ld t2,-8(tp) # t2=dtv
> >>>>
> >>>>      ld a0,8(a0)  # a0=&{modidx,off}
> >>>>      ld t1,8(a0)  # t1=off
> >>>>      ld a0,(a0)   # a0=modidx
> >>>>      sll a0,a0,3  # a0=8*modidx
> >>>>
> >>>>      add a0,a0,t2 # a0=dtv+8*modidx
> >>>>      ld a0,(a0)   # a0=dtv[modidx]
> >>>>      add a0,a0,t1 # a0=dtv[modidx]+off
> >>>>      sub a0,a0,tp # a0=dtv[modidx]+off-tp
> >>>>
> >>>>      ld t1,(sp)
> >>>>      ld t2,8(sp)
> >>>>      add sp,sp,8
> >>>>      jr t0
> >>>
> >>> Any feedback on this? Offhand, it looks like adjusting sp by 8 is
> >>> wrong and that should be 16. Anything else? Does anyone have recent
> >>> enough tooling to test this?
> >>
> >> Tatsuyuki, do you have links to the latest version of
> >> gcc/binutils/glibc patches?
> >> Downloading patches from these mailing lists is probably a large
> >> hurdle for many users, so having the relevant repositories online may
> >> help.
> >>
> >> mold has implemented RISC-V TLSDESC.
> >>
> >> On the LLVM side, I have reviewed
> >> https://github.com/llvm/llvm-project/pull/66915 and am waiting for it
> >> to land, before I can check the lld status.
> >
> > To test this, drop it in src/ldso/riscv64/tlsdesc.s, and add to
> > arch/riscv64/reloc.h:
> >
> > #define REL_TLSDESC R_RISCV_TLSDESC
> >
> > or whatever the reloc name is (I don't think it's in elf.h yet so you
> > probably need to either add it there too or just hard-code the number
> > for testing).
> >
> > Updated version with the sp bugfix attached.
>
> The assembly looks fine to me. (It’s nice that musl don’t need to bother with save/restore at all since DTVs are initialized eagerly.)
> For the patches mentioned in the other thread, most tests were done with glibc’s portable testsuite (tst-elf-*).
> If musl has a similar one, you should be able to run it with my GCC / binutils fork (with either --with_tls=desc at configure time or -mtls-dialect=desc at compile time).
>
> Tatsuyuki.
>
> > Rich
> > <tlsdesc.s>
>

I have verified that the patch works using a runtime test under qemu-user.
I use Paul Kirth's pending LLVM codegen/assembly patch and my pending
lld patch:)

https://gist.github.com/MaskRay/7ad19393fdb0834540db6e7b6b02fa56 .
Copied instructions below:

Patch musl
```diff
--- c/arch/riscv64/reloc.h
+++ i/arch/riscv64/reloc.h
@@ -17,6 +17,7 @@
 #define REL_DTPMOD      R_RISCV_TLS_DTPMOD64
 #define REL_DTPOFF      R_RISCV_TLS_DTPREL64
 #define REL_TPOFF       R_RISCV_TLS_TPREL64
+#define REL_TLSDESC     12

 #define CRTJMP(pc,sp) __asm__ __volatile__( \
        "mv sp, %1 ; jr %0" : : "r"(pc), "r"(sp) : "memory" )
diff --git c/src/ldso/riscv64/tlsdesc.s i/src/ldso/riscv64/tlsdesc.s
new file mode 100644
index 00000000..56d1ce89
--- /dev/null
+++ i/src/ldso/riscv64/tlsdesc.s
@@ -0,0 +1,33 @@
+.text
+.global __tlsdesc_static
+.hidden __tlsdesc_static
+.type __tlsdesc_static,%function
+__tlsdesc_static:
+       ld a0,8(a0)
+       jr t0
+
+.global __tlsdesc_dynamic
+.hidden __tlsdesc_dynamic
+.type __tlsdesc_dynamic,%function
+__tlsdesc_dynamic:
+       add sp,sp,-16
+       sd t1,(sp)
+       sd t2,8(sp)
+
+       ld t2,-8(tp) # t2=dtv
+
+       ld a0,8(a0)  # a0=&{modidx,off}
+       ld t1,8(a0)  # t1=off
+       ld a0,(a0)   # a0=modidx
+       sll a0,a0,3  # a0=8*modidx
+
+       add a0,a0,t2 # a0=dtv+8*modidx
+       ld a0,(a0)   # a0=dtv[modidx]
+       add a0,a0,t1 # a0=dtv[modidx]+off
+       sub a0,a0,tp # a0=dtv[modidx]+off-tp
+
+       ld t1,(sp)
+       ld t2,8(sp)
+       add sp,sp,16
+       jr t0
+
```

```sh
(mkdir -p out/rv64 && cd out/rv64 && ../../configure
--target=riscv64-linux-gnu && make -j 50)
```

Adjust `~/musl/out/rv64/lib/musl-gcc.specs` and update
`~/musl/out/rv64/obj/musl-gcc`
```sh
cat > ~/musl/out/rv64/obj/musl-gcc <<eof
#!/bin/sh
exec "${REALGCC:-riscv64-linux-gnu-gcc}" "$@" -specs
~/musl/out/rv64/lib/musl-gcc.specs
eof
```

Use the test at the end of
https://maskray.me/blog/2021-02-14-all-about-thread-local-storage
```sh
cat > ./a.c <<eof
#include <assert.h>
int foo();
int bar();
int main() {
  assert(foo() == 2);
  assert(foo() == 4);
  assert(bar() == 2);
  assert(bar() == 4);
}
eof

cat > ./b.c <<eof
#include <stdio.h>
__thread int tls0;
extern __thread int tls1;
int foo() { return ++tls0 + ++tls1; }
static __thread int tls2, tls3;
int bar() { return ++tls2 + ++tls3; }
eof

echo '__thread int tls1;' > ./c.c

sed 's/        /\t/' > ./Makefile <<'eof'
.MAKE.MODE = meta curDirOk=true

CC := ~/musl/out/rv64/obj/musl-gcc -O1 -g -fpic -Bbin -fuse-ld=lld
LDFLAGS := -Wl,-rpath=.

all: a0 a1 a2

run: all
        ./a0 && ./a1 && ./a2

b.o: bb.s
        /tmp/Rel/bin/clang --target=riscv64-linux -c bb.s -o $@
c.so: c.o; ${LINK.c} -shared $> -o $@
bc.so: b.o c.o; ${LINK.c} -shared $> -o $@
b.so: b.o c.so; ${LINK.c} -shared $> -o $@

a0: a.o b.o c.o; ${LINK.c} $> -o $@
a1: a.o b.so; ${LINK.c} $> -o $@
a2: a.o bc.so; ${LINK.c} $> -o $@
eof
```


Compile b.c to bb.s. Replace general dynamic code sequences (e.g.
`la.tls.gd a0,tls0; call __tls_get_addr@plt`) with TLSDESC, e.g.
```
.Ltlsdesc_hi0:
  auipc a0, %tlsdesc_hi(tls0)
  ld  a1, %tlsdesc_load_lo(.Ltlsdesc_hi0)(a0)
  addi  a0, a0, %tlsdesc_add_lo(.Ltlsdesc_hi0)
  jalr  t0, 0(a1), %tlsdesc_call(.Ltlsdesc_hi0)
  add   a0, a0, tp
```

Apply LLVM CodeGen/assembly and lld patch
(https://github.com/llvm/llvm-project/pull/79099) to llvm-project.
Build clang and lld.
Create an alias `bin/ld.lld` to be used with `-Bbin -fuse-ld=lld`.

`bmake run` => succeeded!

% bmake run
~/musl/out/rv64/obj/musl-gcc -O1 -g -fpic -Bbin -fuse-ld=lld -g  -c a.c
/tmp/Rel/bin/clang --target=riscv64-linux -c bb.s -o b.o
~/musl/out/rv64/obj/musl-gcc -O1 -g -fpic -Bbin -fuse-ld=lld -g  -c c.c
~/musl/out/rv64/obj/musl-gcc -O1 -g -fpic -Bbin -fuse-ld=lld -g
-Wl,-rpath=. a.o b.o c.o -o a0
~/musl/out/rv64/obj/musl-gcc -O1 -g -fpic -Bbin -fuse-ld=lld -g
-Wl,-rpath=. -shared c.o -o c.so
~/musl/out/rv64/obj/musl-gcc -O1 -g -fpic -Bbin -fuse-ld=lld -g
-Wl,-rpath=. -shared b.o c.so -o b.so
~/musl/out/rv64/obj/musl-gcc -O1 -g -fpic -Bbin -fuse-ld=lld -g
-Wl,-rpath=. a.o b.so -o a1
~/musl/out/rv64/obj/musl-gcc -O1 -g -fpic -Bbin -fuse-ld=lld -g
-Wl,-rpath=. -shared b.o c.o -o bc.so
~/musl/out/rv64/obj/musl-gcc -O1 -g -fpic -Bbin -fuse-ld=lld -g
-Wl,-rpath=. a.o bc.so -o a2
./a0 && ./a1 && ./a2