[LLVMbugs] [Bug 964] NEW: switch lowering makes ugly code for a simple scanning loop

[bugzilla-daemon at cs.uiuc.edu]

http://llvm.org/bugs/show_bug.cgi?id=964

           Summary: switch lowering makes ugly code for a simple scanning
                    loop
           Product: libraries
           Version: 1.8
          Platform: All
        OS/Version: All
            Status: NEW
          Severity: normal
          Priority: P2
         Component: Common Code Generator Code
        AssignedTo: unassignedbugs at nondot.org
        ReportedBy: sabre at nondot.org


Consider this function (very similar to strhr):

void foo(unsigned char C);
const char *FindChar(const char *CurPtr) {
  unsigned char C;
  do
    C = *CurPtr++;
  while (C != 'x' && C != '\0');

  foo(C);
  return CurPtr;
}

We currently compile the loop to:

LBB1_1: #bb
        movb (%esi), %al
        incl %esi
        cmpb $119, %al
        jg LBB1_4       #bb
LBB1_3: #bb
        testb %al, %al
        je LBB1_2       #bb7
        jmp LBB1_1      #bb
LBB1_4: #bb
        cmpb $120, %al
        jne LBB1_1      #bb

It seems that switch lowering could produce something like:

        cmpb $120, %al
        je out
        testb %al, %al
        jnz LBB1_1       #bb7

which would be much faster.

-Chris



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