Compile following function with options -Os -mthumb -march=armv5te
void bar(char*, char*, int);
void foo(char* left, char* rite, int element)
{
while (left <= rite)
{
rite -= element;
bar(left, rite, element);
left += element;
}
}
Gcc generates:
push {r3, r4, r5, r6, r7, lr}
mov r5, r0
mov r6, r1
mov r7, r2
neg r4, r2 // A
b .L2
.L3:
add r6, r6, r4 // B
mov r0, r5
mov r1, r6
mov r2, r7
bl bar
add r5, r5, r7
.L2:
cmp r5, r6
bls .L3
@ sp needed for prologue
pop {r3, r4, r5, r6, r7, pc}
Note that instruction A computes (r4 = -r2), and r4 is only used by instruction
B (r6 = r6 + r4), this can be simplified to (r6 = r6 - r7, r7 contains the
original r2). Thus we can reduce one instruction.
Expression rite -= element was transformed to the following by the gimplify
pass
element.0 = (unsigned int) element;
D.2003 = -element.0;
rite = rite + D.2003;
This form was kept until pass neg.c.156r.loop2_invariant. Then the expression
-element was identified as loop invariant and hoisted out of the loop. So
caused the current result.
Is the transform of gimplify intended? Do we have any chance to merge the
previous expressions back to (rite = rite - element) before loop invariant
pass?
--
Summary: redundant neg instruction caused by loop-invariant
Product: gcc
Version: 4.5.0
Status: UNCONFIRMED
Severity: normal
Priority: P3
Component: target
AssignedTo: unassigned at gcc dot gnu dot org
ReportedBy: carrot at google dot com
GCC build triplet: i686-linux
GCC host triplet: i686-linux
GCC target triplet: arm-eabi
http://gcc.gnu.org/bugzilla/show_bug.cgi?id=40815
