http://llvm.org/bugs/show_bug.cgi?id=11175
Summary: LoopRotation pass too restrictive
Product: libraries
Version: trunk
Platform: All
OS/Version: All
Status: NEW
Severity: normal
Priority: P
Component: Loop Optimizer
AssignedTo: [email protected]
ReportedBy: [email protected]
CC: [email protected]
Created an attachment (id=7477)
--> (http://llvm.org/bugs/attachment.cgi?id=7477)
Test case
Attached is a test case that illustrates how GCC generates dramatically
superior code for a loop for ARM. LLVM could do something comparable if the
LoopRotation pass would not reject loops that have more than one exit. If
LoopRotation would properly handle such loops, then induction-variable analysis
and LICM would have more opportunities to clean things up (as is apparently
done in GCC).
Compiled with -O3, the LLVM generates the following loop code of 10
instructions:
.LBB0_1: @ %while.cond.i
@ =>This Inner Loop Header: Depth=1
add r1, r0, #5
cmp r1, #1
mov r1, #0
blt .LBB0_3
ldr r2, [r4, -r0, lsl #2]
ldr r3, [r5, -r0, lsl #2]
sub r0, r0, #1
mov r1, #1
tst r3, r2
beq .LBB0_1
By contrast GCC generates the following 6-instruction loop:
.L3:
ldr r0, [r3], #4
ldr r1, [r2, #4]!
tst r0, r1
bne .L4
cmp r3, ip
bne .L3
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