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* optimization/10050: Weak optimization on array references in conditiona blocks
@ 2003-03-12 21:06 tm_gccmail
0 siblings, 0 replies; only message in thread
From: tm_gccmail @ 2003-03-12 21:06 UTC (permalink / raw)
To: gcc-gnats
>Number: 10050
>Category: optimization
>Synopsis: Weak optimization on array references in conditiona blocks
>Confidential: no
>Severity: serious
>Priority: medium
>Responsible: unassigned
>State: open
>Class: pessimizes-code
>Submitter-Id: net
>Arrival-Date: Wed Mar 12 21:06:00 UTC 2003
>Closed-Date:
>Last-Modified:
>Originator: Toshi Morita
>Release: unknown-1.0
>Organization:
>Environment:
i386-linux cross sh-elf, but probably all targets
>Description:
Given these functions using the options -O2 -m4 -S:
int flag;
int func1(char *a)
{
int b[3];
b[0] = func();
b[1] = b[0] * 2;
a[0] = b[0];
a[1] = b[1];
return a[0] + b[1] + a[1];
}
int func2(int *a)
{
int b[3];
b[0] = func();
b[1] = b[0] * 2;
if (flag) {
a[0] = b[0];
a[1] = b[1];
}
return a[0] + b[1] + a[1];
}
int func3(int *a)
{
int b[3];
int temp0, temp1;
b[0] = func();
b[1] = b[0] * 2;
temp0 = a[0];
temp1 = a[1];
if (flag) {
temp0 = b[0];
temp1 = b[1];
}
a[0] = temp0;
a[1] = temp1;
return a[0] + b[1] + a[1];
}
GCC is able to keep b[x] in registers for func1:
_func1:
mov.l r8,@-r15
mov r4,r8
mov.l r14,@-r15
sts.l pr,@-r15
mov.l .L2,r0
add #-12,r15
jsr @r0
mov r15,r14
mov r8,r1
mov.b r0,@r8
mov r0,r2
add #1,r1
add r2,r2
mov.b r2,@r1
add #12,r14
mov.b @r8,r0
add r2,r0
exts.b r2,r2
add r2,r0
mov r14,r15
lds.l @r15+,pr
mov.l @r15+,r14
rts
However, gcc generates fairly weak code for func2. It is unable
to retain the values in registers and generates memory writes
within the conditional block:
_func2:
mov.l r8,@-r15
mov r4,r8
mov.l r14,@-r15
sts.l pr,@-r15
mov.l .L7,r0
add #-12,r15
jsr @r0
mov r15,r14
mov.l .L8,r1
mov r0,r2
add r2,r2
mov.l r0,@r14
mov.l @r1,r1
tst r1,r1
bt/s .L5
mov.l r2,@(4,r14) <- (delay slot)
mov.l r0,@r8 <- conditional writes
mov.l r2,@(4,r8) <- conditional writes
.L5:
mov.l @(4,r14),r1
add #12,r14
mov.l @r8,r0
add r1,r0
mov.l @(4,r8),r1
add r1,r0
mov r14,r15
lds.l @r15+,pr
mov.l @r15+,r14
rts
mov.l @r15+,r8
To elicit the desired assembly output where no writes are emitted
in the conditional block, it is necessary to hand-massage the
C source to something like func3, which gives this code:
func3:
mov.l r8,@-r15
mov r4,r8
mov.l r14,@-r15
sts.l pr,@-r15
mov.l .L12,r0
add #-12,r15
jsr @r0
mov r15,r14
mov.l .L13,r1
mov r0,r7
add r7,r7
mov r0,r3
mov.l @r1,r1
mov.l r0,@r14
tst r1,r1
mov.l r7,@(4,r14)
mov.l @r8,r2
bt/s .L11
mov.l @(4,r8),r0 (delay slot)
mov r3,r2 <- no memory write
mov r7,r0 <- no memory write
.L11:
mov.l r2,@r8
add r7,r2
mov.l r0,@(4,r8)
add #12,r14
add r2,r0
mov r14,r15
lds.l @r15+,pr
mov.l @r15+,r14
rts
mov.l @r15+,r8
Toshi
>How-To-Repeat:
See above.
>Fix:
Don't know.
>Release-Note:
>Audit-Trail:
>Unformatted:
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