[LLVMbugs] [Bug 15187] New: logic function not optimized

bugzilla-daemon at llvm.org bugzilla-daemon at llvm.org
Wed Feb 6 14:37:51 PST 2013


            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: unassignedbugs at nondot.org
          Reporter: matic at nimp.co.uk
                CC: llvmbugs at cs.uiuc.edu
    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;
        out ^= in[i] & in[(i+1)%8];
    }//expand to out =
    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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