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[Bug rtl-optimization/43908] New: Very poor code generation (unnecessary conditionals and reloads) for ARM
- From: "tobias at ringis dot se" <gcc-bugzilla at gcc dot gnu dot org>
- To: gcc-bugs at gcc dot gnu dot org
- Date: 27 Apr 2010 12:05:56 -0000
- Subject: [Bug rtl-optimization/43908] New: Very poor code generation (unnecessary conditionals and reloads) for ARM
- Reply-to: gcc-bugzilla at gcc dot gnu dot org
The following code:
struct A { int r0; int r1; };
void sigh(struct A *a, const int d)
{
int i;
for (i = 0; i < 8; ++i) {
if (d & (1U << i))
a->r0 = 1;
else
a->r1 = 1;
}
}
compiled using
arm-none-eabi-gcc -mcpu=arm7tdmi -marm -O3 -S -o- tmp2.c
produces very poor code:
sigh:
@ Function supports interworking.
@ args = 0, pretend = 0, frame = 0
@ frame_needed = 0, uses_anonymous_args = 0
@ link register save eliminated.
tst r1, #1
moveq r3, #1
movne r3, #1
streq r3, [r0, #4]
strne r3, [r0, #0]
tst r1, #2
moveq r3, #1
movne r3, #1
streq r3, [r0, #4]
strne r3, [r0, #0]
tst r1, #4
moveq r3, #1
movne r3, #1
streq r3, [r0, #4]
strne r3, [r0, #0]
tst r1, #8
moveq r3, #1
movne r3, #1
streq r3, [r0, #4]
strne r3, [r0, #0]
tst r1, #16
moveq r3, #1
movne r3, #1
streq r3, [r0, #4]
strne r3, [r0, #0]
tst r1, #32
moveq r3, #1
movne r3, #1
streq r3, [r0, #4]
strne r3, [r0, #0]
tst r1, #64
moveq r3, #1
movne r3, #1
streq r3, [r0, #4]
strne r3, [r0, #0]
tst r1, #128
movne r3, #1
moveq r3, #1
strne r3, [r0, #0]
streq r3, [r0, #4]
bx lr
Note the silly occurrences of:
moveq r1, #1
movne r1, #1
More importantly, there is no need to load the constant 1 into r3 in every
iteration. It should be loaded only once before the (unrolled) loop.
--
Summary: Very poor code generation (unnecessary conditionals and
reloads) for ARM
Product: gcc
Version: 4.5.0
Status: UNCONFIRMED
Severity: normal
Priority: P3
Component: rtl-optimization
AssignedTo: unassigned at gcc dot gnu dot org
ReportedBy: tobias at ringis dot se
GCC host triplet: x86_64-redhat-linux
GCC target triplet: arm-none-eabi
http://gcc.gnu.org/bugzilla/show_bug.cgi?id=43908