https://llvm.org/bugs/show_bug.cgi?id=23538
Bug ID: 23538
Summary: ExitValue rewrite in IndVarSimplify introduced high
cost operations in loop preheader
Product: libraries
Version: trunk
Hardware: PC
OS: Linux
Status: NEW
Severity: normal
Priority: P
Component: Loop Optimizer
Assignee: [email protected]
Reporter: [email protected]
CC: [email protected]
Classification: Unclassified
Created attachment 14333
--> https://llvm.org/bugs/attachment.cgi?id=14333&action=edit
testcase 1.cc
ExitValue rewrite in IndVarSimplify may introduce stmt (like div) with high
expansion cost in loop preheader. Existing implementation doesn't consider the
cost. If the loop is not hot enough or ExitValue rewrite doesn't reduce the
amount of IV processing in loop substantially, the high expansion cost in loop
preheader will be a problem.
The simple testcase 1.cc attached shows the problem.
~/workarea/llvm-r236609/build/bin/clang++ -O2 -S 1.cc
# BB#1: # %while.body.lr.ph
-----------
leal -12(%rsi), %r8d
movl $2863311531, %ecx # imm = 0xAAAAAAAB
imulq %r8, %rcx
shrq $35, %rcx
shll $2, %ecx
leal (%rcx,%rcx,2), %ecx
subl %ecx, %r8d
-----------
movl $-1640531527, %ecx # imm = 0xFFFFFFFF9E3779B9
.align 16, 0x90
.LBB0_2: # %while.body
# =>This Inner Loop Header: Depth=1
movl (%rdi), %edx
leal (%rdx,%rdx), %eax
andl $16843008, %eax # imm = 0x1010100
addl %ecx, %edx
subl %eax, %edx
movl %edx, 4(%rsp)
addq $12, %rdi
addl $-12, %esi
cmpl $11, %esi
movl %edx, %ecx
ja .LBB0_2
The insns between the two dash lines are the extra insns generated for ExitVal
rewrite in IndVarSimplify.
It is not easy to evaluate whether ExitValue rewrite will or will not reduce
the amount of IV processing in loop in IndVarSimplify pass. Without profiling
information support, it is also difficult to know for sure whether a loop is
hot enough. So it may be good to be a little conservative here at least in
none-FDO mode (Feedback Optimization mode).
I think ExitValue rewrite should use the similar logic as
LinearFunctionTestReplace which rewrites ICMP to ICMPZero only when expansion
cost is low.
The problem degraded one of our internal benchmark by 10% and another one by
1.5%. We didn't see >1% regression by simply turning off ExitValue rewrite.
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