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Date: Fri, 08 Nov 2013 09:28:00 -0000
Message-ID: <CADn89gQ=QP=qJdOafJhwsNs8uPaEjjs2wH8py6aGhj4beLBCmQ@mail.gmail.com>
Subject: How can I tune gcc to move up simple common subexpression?
From: Konstantin Vladimirov <konstantin.vladimirov@gmail.com>
To: gcc@gcc.gnu.org, gcc-help@gcc.gnu.org
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Hi,

Consider simple code:

typedef struct
{
  unsigned prev;
  unsigned next;
} foo_t;

void
foo( unsigned x, unsigned y)
  {
    foo_t *ptr = (foo_t *)((void *)x);

    if (y != 0)
      {
         ptr->prev = y;
         ptr->next = x;
       }
     else
       {
         ptr->prev = 0; /* or explicitly ptr->prev = y; no difference */
         ptr->next = 0;
       }
}

GCC 4.7.2 and 4.8.1 both on O2 and Os creates code like:

testl %esi, %esi
movl %edi, %eax
jne .L5
movl $0, (%edi)
movl $0, 4(%rax)
ret
.L5:
movl %esi, (%edi)
movl %edi, 4(%rax)
ret

Which can be obviously changed to:

testl %esi, %esi
movl %edi, %eax
movl %esi, (%edi)
jne .L5
movl $0, 4(%rax)
ret
.L5:
movl %edi, 4(%rax)
ret

May be there are some options to make it behave so? This is question
for gcc-help group.

Question for gcc group is trickier:

May be in x86 it is not a big deal, but I am working on my private
backend, that have predicated instructions and second form is really
much more prefferable. May be I can somehow tune my backend to achieve
this effect? I can see that 210r.csa pass (try_optimize_cfg before it,
not pass itself) can move code upper in  some cases, but only with
very simple memory addressing and rather unstable, say changing

         ptr->prev = y;
         ptr->next = x;

to

         ptr->prev = x;
         ptr->next = y;

may break everything just because next is second member and addressed
like M[%r+4].

Any ideas?

---
With best regards, Konstantin