http://llvm.org/bugs/show_bug.cgi?id=20701
Bug ID: 20701
Summary: TwoAddressInstructionPass fails to optimize mov+add to
lea
Product: libraries
Version: trunk
Hardware: PC
OS: Linux
Status: NEW
Severity: normal
Priority: P
Component: Common Code Generator Code
Assignee: [email protected]
Reporter: [email protected]
CC: [email protected]
Classification: Unclassified
Created attachment 12916
--> http://llvm.org/bugs/attachment.cgi?id=12916&action=edit
test case
I am not an expert on codegen, but it looks a misoptimization to me.
The test case (b.ll) is attached.
llc b.ll -mtriple=i386-apple-darwin -o -
gives
movl %esi, %eax
movl %edi, %ecx
testb $1, 4(%esp)
je LBB0_2
## BB#1: ## %then
addl $16, %eax
retl
LBB0_2: ## %else
addl $32, %ecx
movl %ecx, %eax
retl
where I believe the addl+movl in LBB0_2 should be combined to an leal.
The problematic machine code pass is TwoAddressInstructionPass. The machine
code for BB#2 before this pass is:
BB#2: derived from LLVM BB %else
Predecessors according to CFG: BB#0
%vreg3<def,tied1> = ADD32ri8 %vreg6<kill,tied0>, 32,
%EFLAGS<imp-def,dead>; GR32:%vreg3,%vreg6
%vreg9<def> = COPY %vreg3<kill>; GR32:%vreg9,%vreg3
Successors according to CFG: BB#3
ConvertInstTo3Addr (TwoAddressInstructionPass.cpp:1159) is not invoked because
vreg6 is killed and isProfitableToConv3Addr returns false. As a result, the
machine code afterwards is:
BB#2: derived from LLVM BB %else
Predecessors according to CFG: BB#0
%vreg3<def> = COPY %vreg6<kill>; GR32:%vreg3,%vreg6
%vreg3<def,tied1> = ADD32ri8 %vreg3<tied0>, 32, %EFLAGS<imp-def,dead>;
GR32:%vreg3
%vreg9<def> = COPY %vreg3<kill>; GR32:%vreg9,%vreg3
Successors according to CFG: BB#3
without combining ADDR32ri8 and COPY to LEA32r.
I am not familiar with this pass. So it would be awesome if anyone can help me
figure out the root cause and discuss how to fix this issue (if it is really an
issue at first hand).
Thanks,
Jingyue
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