Compile the attached source code with options -march=armv5te -mthumb -Os, I got
push {r4, lr}
ldr r4, [r0, #8]
ldr r3, [r0, #4]
b .L2
.L7:
ldr r2, [r3, #8]
ldr r1, [r2]
ldr r2, [r3]
add r2, r1, r2
ldr r1, [r3, #4]
ldr r1, [r1]
sub r2, r2, r1
ldr r1, [r3, #12]
cmp r1, #1
beq .L4
cmp r1, #2
bne .L3
b .L12 // C
.L4: // ---------BEGIN BLOCK B
ldr r1, [r0]
neg r1, r1
cmp r2, r1
bge .L3
b .L9 // ----------END BLOCK B
.L12: // ---------------BEGIN BLOCK A-------
ldr r1, [r0]
cmp r2, r1
bgt .L9
.L3:
add r3, r3, #16
.L2:
cmp r3, r4
bcc .L7
mov r0, #0
b .L6 // -----------------END BLOCK A---------
.L9:
mov r0, #1
.L6:
@ sp needed for prologue
pop {r4, pc}
If we change the order of block A and block B, we can remove 2 branch
instructions, inst C and another inst at the end of block B.
Need new basic block reorder algorithm for code size optimization?
--
Summary: missed space optimization related to basic block reorder
Product: gcc
Version: 4.5.0
Status: UNCONFIRMED
Severity: normal
Priority: P3
Component: middle-end
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=41396
