http://llvm.org/bugs/show_bug.cgi?id=15187
Bug ID: 15187
Summary: logic function not optimized
Product: new-bugs
Version: 3.2
Hardware: PC
OS: Linux
Status: NEW
Severity: enhancement
Priority: P
Component: new bugs
Assignee: [email protected]
Reporter: [email protected]
CC: [email protected]
Classification: Unclassified
Created attachment 9973
--> http://llvm.org/bugs/attachment.cgi?id=9973&action=edit
C source file to reproduce the problem, IAR compiler output, LLVM 3-2 output
I gave the following function to IAR compiler (targeting CortexM0) and to
clang/LLVM 3.2 (clang -O3 -target thumbv6-eabi -emit-llvm)
int calleeSave8(int in[]){
int out=0;
int i;
for(i=0;i<8;i++){
out ^= in[i] & in[(i+1)%8];
}//expand to out =
(in[0]&in[1])^(in[1]&in[2])^(in[2]&in[3])^(in[3]&in[4])^(in[4]&in[5])^(in[5]&in[6])^(in[6]&in[7])^(in[7]&in[0])
return out;
}
In such case, IAR compiler is able to factor out half of the and operations so
it performs 4 and + 7 xor, LLVM factored only one and operation so it performs
7 and + 7 xor. (I looked at IR code and assembly output)
same thing with Cortex-M3 (clang -O3 -target thumbv7-eabi -emit-llvm ...; llc
... -debug -O3 -code-model=small -march=thumb -mcpu=cortex-m3 ...)
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