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From: "tuliom at linux dot vnet.ibm.com" <gcc-bugzilla@gcc.gnu.org>
To: gcc-bugs@gcc.gnu.org
Subject: [Bug target/67281] New: HTM builtins aren't treated as compiler barriers on powerpc
Date: Wed, 19 Aug 2015 17:22:00 -0000
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https://gcc.gnu.org/bugzilla/show_bug.cgi?id=67281

            Bug ID: 67281
           Summary: HTM builtins aren't treated as compiler barriers on
                    powerpc
           Product: gcc
           Version: 4.9.0
            Status: UNCONFIRMED
          Severity: normal
          Priority: P3
         Component: target
          Assignee: unassigned at gcc dot gnu.org
          Reporter: tuliom at linux dot vnet.ibm.com
  Target Milestone: ---

Depending on the level of optimization, GCC moves a memory access outside a
transaction which breaks the atomicity of the transaction.

This is a small testcase that reproduces this behavior:

$ cat tbegin-barrier.c
long
foo (long dest, long src0, long src1, long tries)
{
  long i;
  for (i = 0; i < tries; i++)
    {
      __builtin_tbegin (0);
      dest = src0 + src1;
      __builtin_tend (0);
    }
  return dest;
}

compiling using: -O2 -S -mcpu=power8 tbegin-barrier.c
gives
foo:
        cmpdi 0,6,0
        blelr 0
        mtctr 6
        add 3,4,5
        .p2align 4,,15
.L3:
        tbegin. 0
        tend. 0
        bdnz .L3
        blr