[PATCH] D34822: [LVI] Constant-propagate a zero extension of the switch condition value through case edges

[Hiroshi Yamauchi via Phabricator via llvm-commits]

yamauchi added a comment.

For more context:

Here's a test C program that demonstrates this issue (reduced from the Python 2.7 runtime eval loop code) :

  void bar(int v);
  
  int foo(unsigned char* p, int (*f)(int)) {
    int op;
    void* targets[256];
    int i;
    for (i = 0; i < 256; ++i) {
      targets[i] = &&unknown_op;
    }
    targets[93] = &&TARGET_93;
    targets[145] = &&TARGET_145;
    for (;;) {
      op = *p++;
   dispatch_op:
      switch (op) {
        TARGET_93:
        op = 93;
        case 93: {
          if (op != 93) {
            f(42);
          }
          goto *targets[*p++];
        }
  
        TARGET_145:
        op = 145;
        case 145: {
          op = *p++;
          goto dispatch_op;
        }
  
        unknown_op:
        default:
          bar(op);
          break;
      }
    }
    return 0;
  }

There's a control merge between "TARGET_93" (that the computed goto may jumps to) and "case 93" (that the normal switch control flow may jump to).

The value of 'op' at "case 93:" would be constant 93 regardless of the control flow.

So, we could eliminate the call to 'f'.

GCC does this whereas LLVM does not.


https://reviews.llvm.org/D34822