[4/4][PATCH] Discussing PR83507

Jeff Law law@redhat.com
Tue Apr 30 15:14:00 GMT 2019

On 4/29/19 10:28 AM, Roman Zhuykov wrote:
> Hi, Segher
>>>>> In pr84524.c we got a loop with an extended inline asm:
>>>>> asm volatile ("" : "+r" (v))
>>>>> which also gives us a “surprising” situation Alexander predicts.
>>>>> For sched-deps scanner such volatile asm is a “true barrier” and we
>>>>> create dependencies to almost all other instructions, while DF scanners
>>>>> don’t give us such information.
>>>> There is no such thing as a "true barrier" in extended asm.  The only thing
>>>> volatile asm means is that the asm has a side effect unknown to the compiler;
>>>> this can *not* be a modification of a register or of memory contents, such
>>>> things are known by the compiler, that's what clobbers and "memory" clobber
>>>> are about.
>>> In sched-deps.c we got:
>>>     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;
>>>        ...
>> 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.
Also note that the integration of the Haifa scheduler was a large
change.  A small detail like this could have been missed.


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