Hi!
On Mon, Apr 29, 2019 at 07:28:12PM +0300, Roman Zhuykov wrote:
> > This code was added in 1997 (r14770). In 2004 the documentation was
> > changed to clarify how things really work (r88999):
> >
> > "Note that even a volatile @code{asm} instruction can be moved relative to
> > other code, including across jump instructions."
> >
> > (followed by an example exactly about what this means for FPU control).
>
> Thanks for pointing to that changes! Unfortunately, sched-deps.c was
> more conservative this 15 years...
> Let’s try to fix it.
If it causes problems now, that would be a good idea yes :-)
> > I mean have the modulo scheduler implement the correct asm semantics, not
> > some more restrictive thing that gets it into conflicts with DF, etc.
> >
> > I don't think this will turn out to be a problem in any way. Some invalid
> > asm will break, sure.
>
> I have started with applying this without any SMS changes:
>
> diff --git a/gcc/sched-deps.c b/gcc/sched-deps.c
> --- a/gcc/sched-deps.c
> +++ b/gcc/sched-deps.c
> @@ -2753,22 +2753,14 @@ sched_analyze_2 (struct deps_desc *deps, rtx
> x, rtx_insn *insn)
>
> case UNSPEC_VOLATILE:
> flush_pending_lists (deps, insn, true, true);
> +
> + if (!DEBUG_INSN_P (insn))
> + reg_pending_barrier = TRUE_BARRIER;
> /* FALLTHRU */
>
> case ASM_OPERANDS:
> case ASM_INPUT:
> {
> - /* Traditional and volatile asm instructions must be considered to use
> - and clobber all hard registers, all pseudo-registers and all of
> - memory. So must TRAP_IF and UNSPEC_VOLATILE operations.
> -
> - Consider for instance a volatile asm that changes the fpu rounding
> - mode. An insn should not be moved across this even if it only uses
> - pseudo-regs because it might give an incorrectly rounded result.
> */
> - if ((code != ASM_OPERANDS || MEM_VOLATILE_P (x))
> - && !DEBUG_INSN_P (insn))
> - reg_pending_barrier = TRUE_BARRIER;
> -
> /* For all ASM_OPERANDS, we must traverse the vector of input
> operands.
> We cannot just fall through here since then we would be confused
> by the ASM_INPUT rtx inside ASM_OPERANDS, which do not indicate
UNSPEC_VOLATILE and volatile asm should have the same semantics, ideally.
One step at a time I guess :-)
> Regstrapping it on x86-64 shows some failures. First is with ms-sysv
> abi test and can be solved like this:
[ snip ]
> Here we have some asms which control stack pointer (sigh!). It
> certainly may be broken at any moment, but right now “memory” clobber
> fixes everything for me.
Makes sense.
> Another failure is:
>
> FAIL: gcc.dg/guality/pr58791-4.c -O2 -DPREVENT_OPTIMIZATION line
> pr58791-4.c:32 i == 486
[ snip ]
> It is connected to debug-info, and I cannot solve it myself. I am not
> sure how this should work when we try to print dead-code variable (i2)
> while debugging -O2 (O3/Os) compiled executable. Jakub created that
> test, he is in CC already.
What does PREVENT_OPTIMIZATION do? It probably needs to be made a bit
stronger.
(It seems to just add some "used"?)
> I will also look at aarch64 regstrap a bit later. But that job should
> also be done for all other targets. Segher, could you test it on
> rs6000?
Do you have an account on the compile farm? It has POWER7 (BE, 32/64),
as well as POWER8 and POWER9 machines (both LE).
https://cfarm.tetaneutral.net/
But I'll do it if you want. Just let me know.
> > > > > In pr84524.c we got a loop with an extended inline asm:
> > > > > asm volatile ("" : "+r" (v))
>
> It seems a good idea to add “memory” clobber into asm and recheck DDG
> in this test on aarch64 with both SMS and sched-deps patches applied.
> I'll check it.
Segher
