[LLVMbugs] [Bug 1160] NEW: loop code badly optimized

bugzilla-daemon at cs.uiuc.edu bugzilla-daemon at cs.uiuc.edu
Fri Feb 2 20:41:52 PST 2007


           Summary: loop code badly optimized
           Product: libraries
           Version: trunk
          Platform: PC
        OS/Version: Linux
            Status: NEW
          Severity: normal
          Priority: P2
         Component: Backend: X86
        AssignedTo: unassignedbugs at nondot.org
        ReportedBy: nicholas at mxc.ca

This small program produces bad x86 assembly code:

unsigned int huge[2048];
unsigned int target[2048];

extern void foo();

int main(int argc, char *argv[])

  for (unsigned int i = 0; i < 2048; i++)
    target[i] |= huge[i];

  unsigned int x = 0;
  for (unsigned int i = 0; i < 2048; i++)
    x += target[i];

  return x;

There are two problems with the resulting x86 assembly.

For the body of the first loop, LLVM produces:

        movl huge(,%eax,4), %ecx
        orl target(,%eax,4), %ecx
        movl %ecx, target(,%eax,4)

when it should really be loading from huge, then or'ing into target.

In the second loop, LLVM mis-predicts the exit branch, which is inexcusable
seeing as the loop is countable:

.LBB1_5:        #bb23
        cmpl $2048, %ecx
        jne .LBB1_3     #bb15   <--- should be reversed
.LBB1_6:        #bb27
        addl $4, %esp
.LBB1_3:        #bb15
        addl target(,%ecx,4), %eax
        incl %ecx
        jmp .LBB1_5     #bb23

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