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* [Bug c/38126] New: suboptimal code for (a && b || !a && !b)
@ 2008-11-15 0:07 sebor at roguewave dot com
2008-11-15 0:11 ` [Bug middle-end/38126] " pinskia at gcc dot gnu dot org
2009-09-12 23:33 ` msebor at gmail dot com
0 siblings, 2 replies; 3+ messages in thread
From: sebor at roguewave dot com @ 2008-11-15 0:07 UTC (permalink / raw)
To: gcc-bugs
I would expect gcc to generate comparable code for both functions below, or
perhaps even better code for foo() than for bar() since the code in foo() is
likely to be more common than the equivalent code in bar(). However, the code
produced for foo() is suboptimal in comparison to the code for bar(). In my
timings on x86 with gcc 4.3.0 at -O2, foo() appears to run about 5% slower than
bar().
$ cat t.c && gcc -S -O2 t.c && cat t.s
int foo (int *a, int *b) { return a && b || !a && !b; }
int bar (int *a, int *b) { return !!a == !!b; }
.file "t.c"
.text
.p2align 4,,15
.globl foo
.type foo, @function
foo:
.LFB2:
testq %rdi, %rdi
je .L2
testq %rsi, %rsi
movl $1, %eax
je .L2
rep
ret
.p2align 4,,10
.p2align 3
.L2:
testq %rdi, %rdi
sete %al
testq %rsi, %rsi
sete %dl
andl %edx, %eax
movzbl %al, %eax
ret
.LFE2:
.size foo, .-foo
.p2align 4,,15
.globl bar
.type bar, @function
bar:
.LFB3:
testq %rdi, %rdi
sete %al
testq %rsi, %rsi
setne %dl
xorl %edx, %eax
movzbl %al, %eax
ret
.LFE3:
.size bar, .-bar
.section .eh_frame,"a",@progbits
.Lframe1:
.long .LECIE1-.LSCIE1
.LSCIE1:
.long 0x0
.byte 0x1
.string "zR"
.uleb128 0x1
.sleb128 -8
.byte 0x10
.uleb128 0x1
.byte 0x3
.byte 0xc
.uleb128 0x7
.uleb128 0x8
.byte 0x90
.uleb128 0x1
.align 8
.LECIE1:
.LSFDE1:
.long .LEFDE1-.LASFDE1
.LASFDE1:
.long .LASFDE1-.Lframe1
.long .LFB2
.long .LFE2-.LFB2
.uleb128 0x0
.align 8
.LEFDE1:
.LSFDE3:
.long .LEFDE3-.LASFDE3
.LASFDE3:
.long .LASFDE3-.Lframe1
.long .LFB3
.long .LFE3-.LFB3
.uleb128 0x0
.align 8
.LEFDE3:
.ident "GCC: (GNU) 4.3.0 20080428 (Red Hat 4.3.0-8)"
.section .note.GNU-stack,"",@progbits
--
Summary: suboptimal code for (a && b || !a && !b)
Product: gcc
Version: 4.3.0
Status: UNCONFIRMED
Severity: normal
Priority: P3
Component: c
AssignedTo: unassigned at gcc dot gnu dot org
ReportedBy: sebor at roguewave dot com
http://gcc.gnu.org/bugzilla/show_bug.cgi?id=38126
^ permalink raw reply [flat|nested] 3+ messages in thread
* [Bug middle-end/38126] suboptimal code for (a && b || !a && !b)
2008-11-15 0:07 [Bug c/38126] New: suboptimal code for (a && b || !a && !b) sebor at roguewave dot com
@ 2008-11-15 0:11 ` pinskia at gcc dot gnu dot org
2009-09-12 23:33 ` msebor at gmail dot com
1 sibling, 0 replies; 3+ messages in thread
From: pinskia at gcc dot gnu dot org @ 2008-11-15 0:11 UTC (permalink / raw)
To: gcc-bugs
--
pinskia at gcc dot gnu dot org changed:
What |Removed |Added
----------------------------------------------------------------------------
Severity|normal |enhancement
Component|c |middle-end
http://gcc.gnu.org/bugzilla/show_bug.cgi?id=38126
^ permalink raw reply [flat|nested] 3+ messages in thread
* [Bug middle-end/38126] suboptimal code for (a && b || !a && !b)
2008-11-15 0:07 [Bug c/38126] New: suboptimal code for (a && b || !a && !b) sebor at roguewave dot com
2008-11-15 0:11 ` [Bug middle-end/38126] " pinskia at gcc dot gnu dot org
@ 2009-09-12 23:33 ` msebor at gmail dot com
1 sibling, 0 replies; 3+ messages in thread
From: msebor at gmail dot com @ 2009-09-12 23:33 UTC (permalink / raw)
To: gcc-bugs
------- Comment #1 from msebor at gmail dot com 2009-09-12 23:33 -------
Code involving bool variables is similarly suboptimal:
$ cat t.cpp && gcc -O2 -S t.cpp && cat t.s
bool foo (bool a, bool b) {
return a && b || !a && !b;
}
bool bar (bool a, bool b) {
return a == b;
}
.file "t.cpp"
.text
.p2align 4,,15
.globl _Z3foobb
.type _Z3foobb, @function
_Z3foobb:
.LFB0:
.cfi_startproc
.cfi_personality 0x3,__gxx_personality_v0
movl %esi, %edx
movl %esi, %eax
xorl $1, %edx
testb %dil, %dil
cmove %edx, %eax
ret
.cfi_endproc
.LFE0:
.size _Z3foobb, .-_Z3foobb
.p2align 4,,15
.globl _Z3barbb
.type _Z3barbb, @function
_Z3barbb:
.LFB1:
.cfi_startproc
.cfi_personality 0x3,__gxx_personality_v0
cmpb %dil, %sil
sete %al
ret
.cfi_endproc
.LFE1:
.size _Z3barbb, .-_Z3barbb
.ident "GCC: (GNU) 4.4.1 20090725 (Red Hat 4.4.1-2)"
.section .note.GNU-stack,"",@progbits
--
http://gcc.gnu.org/bugzilla/show_bug.cgi?id=38126
^ permalink raw reply [flat|nested] 3+ messages in thread
end of thread, other threads:[~2009-09-12 23:33 UTC | newest]
Thread overview: 3+ messages (download: mbox.gz / follow: Atom feed)
-- links below jump to the message on this page --
2008-11-15 0:07 [Bug c/38126] New: suboptimal code for (a && b || !a && !b) sebor at roguewave dot com
2008-11-15 0:11 ` [Bug middle-end/38126] " pinskia at gcc dot gnu dot org
2009-09-12 23:33 ` msebor at gmail dot com
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