http://gcc.gnu.org/bugzilla/show_bug.cgi?id=58017
Bug ID: 58017
Summary: [SH] Use shift and test for unsigned compare
Product: gcc
Version: 4.9.0
Status: UNCONFIRMED
Severity: enhancement
Priority: P3
Component: target
Assignee: unassigned at gcc dot gnu.org
Reporter: olegendo at gcc dot gnu.org
Target: sh*-*-*
On SH unsigned int comparisons against some constants can be converted to shift
and test sequences to potentially save one register.
For example:
int test_00 (unsigned int a, int b, int c)
{
if (a == 0 || a == 1)
return 50;
return 0;
}
compiled with -O2:
mov #1,r1
cmp/hi r1,r4 // T = r4 > r1 (unsigned)
bf/s .L7
mov #0,r0
rts
nop
.L7:
rts
mov #50,r0
can also be written as:
shlr r4 // r4 >>= 1
tst r4,r4 // T = r4 == 0
bt .L7
mov #0,r0
rts
nop
.L7:
rts
mov #50,r0
In this case it doesn't save much, but if register pressure is high and the
compared value is not used anywhere else, this can avoid using one register for
the comparison constant. This is probably only going to be beneficial for
right shift patterns that can be done in 1 or 2 insns and if the inverted
comparison result can be handled by a following conditional branch.
If the comparison constant does not fit into an imm8 it will be loaded from the
constant pool:
mov.l .L7,r1
cmp/hi r1,r4
bf .L3
mov #50,r0
rts
nop
.L3:
rts
mov #0,r0
.L8:
.align 2
.L7:
.long 65535
In this case it might always be beneficial to replace the comparison with
shlr16 r4
tst r4,r4
especially if the compared reg is dead after the comparison.
