Hi! I try to investigate on this problem, and modify the testcase to
compile and run on aarch64 for reference, but I get some strange result
(comment shows the info that I see by stepping through by using gdb):
typedef double __attribute__((vector_size(16))) v2df;
void use1(double d) {}
__attribute__((noipa)) v2df use(double d)
{
//reg v8's value: {1, 2}
register v2df x asm("v8") = {5, 9};
//reg v8's value: {5, 9}
__asm__("" : "+w" (x));
return x;
}
void test(void)
{
register v2df x asm("v8") = {1, 2};
__asm__("" : "+w" (x));
//reg v8's value: {1, 2}
use(x[0]);
//reg v8's value: {1, 0}
use1(x[1]);
}
int main(int argc, char **argv)
{
test();
return 0;
}
The compile command is: gcc -march=armv8-a -Og -g 1.c (gcc
8.3.0+binutils 2.31)
Disassembly of test() and use():
0000000000400558 <use>:
400558: fc1f0fe8 str d8, [sp, #-16]!
40055c: 90000000 adrp x0, 400000 <_init-0x3e0>
400560: 3dc19c08 ldr q8, [x0, #1648]
400564: 4ea81d00 mov v0.16b, v8.16b
400568: fc4107e8 ldr d8, [sp], #16
40056c: d65f03c0 ret
0000000000400570 <test>:
400570: a9be7bfd stp x29, x30, [sp, #-32]!
400574: 910003fd mov x29, sp
400578: fd000be8 str d8, [sp, #16]
40057c: 90000000 adrp x0, 400000 <_init-0x3e0>
400580: 3dc1a008 ldr q8, [x0, #1664]
400584: 5e080500 mov d0, v8.d[0]
400588: 97fffff4 bl 400558 <use>
40058c: fd400be8 ldr d8, [sp, #16]
400590: a8c27bfd ldp x29, x30, [sp], #32
400594: d65f03c0 ret
As the register value in the comments, The compiling output on aarch64
also clobbers the high parts of vector register. I googled for some
documents and I find this:
https://developer.arm.com/documentation/den0024/a/The-ABI-for-ARM-64
-bit-Architecture/Register-use-in-the-AArch64-Procedure-Call-Standard
/Parameters-in-NEON-and-floating-point-registers
Seems ARMv8-A only guarantees to preserve low 64-bit value of
NEON/floating-point register value. I'm not sure that I modify the
testcase in the right way and maybe we need more investigations. Any
ideas or suggestion?
On Wed, 2023-08-16 at 11:27 +0800, Xi Ruoyao wrote:
> The implementation fails to handle this test case properly:
>
> typedef double __attribute__((vector_size(32))) v4df;
>
> void use1(double);
>
> __attribute__((noipa)) double use(double)
> {
> register double x asm("f24") = 114.514;
> __asm__("" : "+f" (x));
> return x;
> }
>
> void test(void)
> {
> register v4df x asm("f24") = {1, 2, 3, 4};
> __asm__("" : "+f" (x));
> use(x[1]);
> use1(x[3]);
> }
>
> Here use() attempts to save and restore f24, but it uses fst.d/fld.d,
> clobbering the high 192 bits of xr24. Now test() passes a wrong
> value
> of x[3] to use1().
>
> Note that saving and restoring f24 with xvst/xvld in use() won't
> really
> fix the issue because in real life use() can be in another
> translation
> unit (or even a shared library) compiled with -mno-lsx. So it seems
> we
> need to tell the compiler "a function call may clobber the high bits
> of
> a vector register even if the corresponding floating-point register
> is
> saved". I'm not sure how to accomplish this...
>
> On Tue, 2023-08-15 at 09:05 +0800, Chenghui Pan wrote:
> > This is an update of:
> > https://gcc.gnu.org/pipermail/gcc-patches/2023-August/626194.html
> >
> > This version of patch set only introduces some small simplications
> > of
> > implementation. Because I missed the size limitation of mail size,
> > the
> > huge testsuite patches of v2 and v3 are not shown in the mail list.
> > So,
> > testsuite patches are splited from this patch set again and will be
> > submitted
> > independently in the future.
> >
> > Binutils-gdb introduced LSX/LASX support since 2.41 release:
> > https://lists.gnu.org/archive/html/info-gnu/2023-07/msg00009.html
> >
> > Brief history of patch set version:
> > v1 -> v2:
> > - Reduce usage of "unspec" in RTL template.
> > - Append Support of ADDR_REG_REG in LSX and LASX.
> > - Constraint docs are appended in gcc/doc/md.texi and ccomment
> > block.
> > - Codes related to vecarg are removed.
> > - Testsuite of LSX and LASX is added in v2. (Because of the size
> > limitation of
> > mail list, these patches are not shown)
> > - Adjust the loongarch_expand_vector_init() function to reduce
> > instruction
> > output amount.
> > - Some minor implementation changes of RTL templates.
> >
> > v2 -> v3:
> > - Revert vabsd/xvabsd RTL templates to unspec impl.
> > - Resolve warning in gcc/config/loongarch/loongarch.cc when
> > bootstrapping
> > with BOOT_CFLAGS="-O2 -ftree-vectorize -fno-vect-cost-model -
> > mlasx".
> > - Remove redundant definitions in lasxintrin.h.
> > - Refine commit info.
> >
> > Lulu Cheng (6):
> > LoongArch: Add Loongson SX vector directive compilation
> > framework.
> > LoongArch: Add Loongson SX base instruction support.
> > LoongArch: Add Loongson SX directive builtin function support.
> > LoongArch: Add Loongson ASX vector directive compilation
> > framework.
> > LoongArch: Add Loongson ASX base instruction support.
> > LoongArch: Add Loongson ASX directive builtin function support.
> >
> > gcc/config.gcc | 2 +-
> > gcc/config/loongarch/constraints.md | 131 +-
> > .../loongarch/genopts/loongarch-strings | 4 +
> > gcc/config/loongarch/genopts/loongarch.opt.in | 12 +-
> > gcc/config/loongarch/lasx.md | 5122
> > ++++++++++++++++
> > gcc/config/loongarch/lasxintrin.h | 5338
> > +++++++++++++++++
> > gcc/config/loongarch/loongarch-builtins.cc | 2686 ++++++++-
> > gcc/config/loongarch/loongarch-c.cc | 18 +
> > gcc/config/loongarch/loongarch-def.c | 6 +
> > gcc/config/loongarch/loongarch-def.h | 9 +-
> > gcc/config/loongarch/loongarch-driver.cc | 10 +
> > gcc/config/loongarch/loongarch-driver.h | 2 +
> > gcc/config/loongarch/loongarch-ftypes.def | 666 +-
> > gcc/config/loongarch/loongarch-modes.def | 39 +
> > gcc/config/loongarch/loongarch-opts.cc | 89 +-
> > gcc/config/loongarch/loongarch-opts.h | 3 +
> > gcc/config/loongarch/loongarch-protos.h | 35 +
> > gcc/config/loongarch/loongarch-str.h | 3 +
> > gcc/config/loongarch/loongarch.cc | 4586
> > +++++++++++++-
> > gcc/config/loongarch/loongarch.h | 117 +-
> > gcc/config/loongarch/loongarch.md | 56 +-
> > gcc/config/loongarch/loongarch.opt | 12 +-
> > gcc/config/loongarch/lsx.md | 4481
> > ++++++++++++++
> > gcc/config/loongarch/lsxintrin.h | 5181
> > ++++++++++++++++
> > gcc/config/loongarch/predicates.md | 333 +-
> > gcc/doc/md.texi | 11 +
> > 26 files changed, 28668 insertions(+), 284 deletions(-)
> > create mode 100644 gcc/config/loongarch/lasx.md
> > create mode 100644 gcc/config/loongarch/lasxintrin.h
> > create mode 100644 gcc/config/loongarch/lsx.md
> > create mode 100644 gcc/config/loongarch/lsxintrin.h
> >
>
