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* [Bug tree-optimization/34417] New: simplify '(x & A) % B' if 'B > A/2'
@ 2007-12-10 10:56 wouter dot vermaelen at scarlet dot be
2007-12-10 13:40 ` [Bug tree-optimization/34417] " rguenth at gcc dot gnu dot org
0 siblings, 1 reply; 3+ messages in thread
From: wouter dot vermaelen at scarlet dot be @ 2007-12-10 10:56 UTC (permalink / raw)
To: gcc-bugs
Hi,
In one of my application I need the expression '(x & 0xf) % 9'. Because of the
restricted range of (x & 0xf) it's possible to replace the modulo operation
with an if (see examples below). It would be nice if gcc could do this
optimization automatically.
unsigned fooA(unsigned x)
{
return (x & 0xf) % 9;
}
unsigned fooB(unsigned x)
{
x &= 0xf;
if (x >= 9) x -= 9;
return x;
}
unsigned fooC(unsigned x)
{
x &= 0xf;
return std::min(x, x - 9);
}
The generated code is pasted below (SVN revision r130738, linux_x86_64, -O3).
Version B and C are very close (maybe C is slightly better) but they are
clearly better than version A.
fooA: andl $15, %edi
movl $954437177, %edx
movl %edi, %eax
mull %edx
shrl %edx
leal 0(,%rdx,8), %eax
addl %edx, %eax
subl %eax, %edi
movl %edi, %eax
ret
fooB: andl $15, %edi
leal -9(%rdi), %eax
cmpl $9, %edi
cmovae %eax, %edi
movl %edi, %eax
ret
fooC: andl $15, %edi
leal -9(%rdi), %eax
cmpl %edi, %eax
cmova %edi, %eax
ret
--
Summary: simplify '(x & A) % B' if 'B > A/2'
Product: gcc
Version: 4.3.0
Status: UNCONFIRMED
Severity: enhancement
Priority: P3
Component: tree-optimization
AssignedTo: unassigned at gcc dot gnu dot org
ReportedBy: wouter dot vermaelen at scarlet dot be
http://gcc.gnu.org/bugzilla/show_bug.cgi?id=34417
^ permalink raw reply [flat|nested] 3+ messages in thread
* [Bug tree-optimization/34417] simplify '(x & A) % B' if 'B > A/2'
2007-12-10 10:56 [Bug tree-optimization/34417] New: " wouter dot vermaelen at scarlet dot be
@ 2007-12-10 13:40 ` rguenth at gcc dot gnu dot org
0 siblings, 0 replies; 3+ messages in thread
From: rguenth at gcc dot gnu dot org @ 2007-12-10 13:40 UTC (permalink / raw)
To: gcc-bugs
------- Comment #1 from rguenth at gcc dot gnu dot org 2007-12-10 13:40 -------
Confirmed.
VRP could do this, though we generally avoid creating control flow and extra
basic blocks here.
--
rguenth at gcc dot gnu dot org changed:
What |Removed |Added
----------------------------------------------------------------------------
CC| |rguenth at gcc dot gnu dot
| |org
Status|UNCONFIRMED |NEW
Ever Confirmed|0 |1
Keywords| |missed-optimization
Last reconfirmed|0000-00-00 00:00:00 |2007-12-10 13:40:29
date| |
http://gcc.gnu.org/bugzilla/show_bug.cgi?id=34417
^ permalink raw reply [flat|nested] 3+ messages in thread
end of thread, other threads:[~2021-07-26 20:45 UTC | newest]
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2012-02-27 4:27 ` [Bug tree-optimization/34417] simplify '(x & A) % B' if 'B > A/2' pinskia at gcc dot gnu.org
2021-07-26 20:45 ` pinskia at gcc dot gnu.org
2007-12-10 10:56 [Bug tree-optimization/34417] New: " wouter dot vermaelen at scarlet dot be
2007-12-10 13:40 ` [Bug tree-optimization/34417] " rguenth at gcc dot gnu dot org
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