Re: x86 branches vs conditional moves

[Steven Bosscher]

On Sat, Jul 8, 2017 at 1:30 AM, Michael Clark wrote:
> Why does gcc branch on _Bool, but emits a conditional move for an integer? 
> can it emit cmovne instead of branching? also curious where one would change 
> this to learn about GCC internals.

The RTL generated for the int and _Bool case is different:

;; Function select_int (select_int, funcdef_no=0, decl_uid=1820,
cgraph_uid=0, symbol_order=0)

    3: NOTE_INSN_BASIC_BLOCK 2
    2: NOTE_INSN_FUNCTION_BEG
    5: flags:CCZ=cmp([`c'],0)
    6: pc={(flags:CCZ==0)?L11:pc}

    7: NOTE_INSN_BASIC_BLOCK 3
    8: r88:SI=[`a']

   11: L11:
   12: NOTE_INSN_BASIC_BLOCK 4
   13: r88:SI=[`b']

   18: L18:
   21: NOTE_INSN_BASIC_BLOCK 5
   19: ax:SI=r88:SI
   20: use ax:SI


;; Function select_bool (select_bool, funcdef_no=1, decl_uid=1822,
cgraph_uid=1, symbol_order=1)

    3: NOTE_INSN_BASIC_BLOCK 2
    2: NOTE_INSN_FUNCTION_BEG
    5: flags:CCZ=cmp([`C'],0)
    6: pc={(flags:CCZ==0)?L13:pc}

    7: NOTE_INSN_BASIC_BLOCK 3
    8: flags:CCZ=cmp([`a'],0)
    9: r90:QI=flags:CCZ!=0

   13: L13:
   14: NOTE_INSN_BASIC_BLOCK 4
   15: flags:CCZ=cmp([`b'],0)
   16: r90:QI=flags:CCZ!=0

   22: L22:
   25: NOTE_INSN_BASIC_BLOCK 5
   23: ax:QI=r90:QI
   24: use ax:QI

As noted earlier, this causes RTL if-conversion to fail.

Somewhat related to PR20070, but even with -fcrossjumping, the test
case fails. (I expected cross-jumping to merge the
"r90:QI=flags:CCZ!=0" insns.)

Ciao!
Steven
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