http://gcc.gnu.org/bugzilla/show_bug.cgi?id=56096
Bug #: 56096
Summary: Bad code generated for conditional shift
Classification: Unclassified
Product: gcc
Version: 4.7.2
Status: UNCONFIRMED
Severity: normal
Priority: P3
Component: target
AssignedTo: unassig...@gcc.gnu.org
ReportedBy: til...@code-monkey.de
Compiling this snippet
unsigned f1 (unsigned x, unsigned m)
{
x >>= ((m & 0x008080) ? 8 : 0);
return x;
}
with gcc 4.7.2 gives this code for ARMv5:
$ armv5tel-softfloat-linux-gnueabi-gcc -O2 -S -o- f.c
[...]
ldr r3, .L4
and r3, r1, r3
cmp r3, #0
movne r3, #8 @ XXX
moveq r3, #0 @ XXX
mov r0, r0, lsr r3 @ XXX
bx lr
[...]
Those three mov instructions are clearly sub-optimal.
Replacing the ternary operator with an if-statement gives the expected code
sequence:
unsigned f1 (unsigned x, unsigned m)
{
if (m & 0x008080)
x >>= 8;
return x;
}
->
ldr r3, .L6
and r3, r1, r3
cmp r3, #0
movne r0, r0, lsr #8
bx lr
ie we saved two mov instructions.
