[PATCH] D62190: [RISCV] Allow shrink wrapping for RISC-V

[Lewis Revill via Phabricator via llvm-commits]

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So I've been looking into the benchmarks that I've been running to figure out why code doesn't change with this enabled.

I found that GCC's shrink wrapping does make a difference, and that comes from a loop which looks like the following:

  int foo(int rpt) {
    int i;
    int r;
  
    for (i = 0; i < rpt; ++i) {
      srand(0);
      r = ...;
    }
  
    return r;
  }

GCC appears to convert this into the equivalent of:

  int foo(int rpt) {
    if (rpt == 0)
      return 0;
  
    int i;
    int r;
  
    for (i = 0; i < rpt; ++i) {
      srand(0);
      r = ...;
    }
  
    return r;
  }

And is then able to shrink wrap around the if statement. Sure enough, if I feed the modified function above into LLVM, we are able to shrink wrap it as well.

(I'm not certain if this conversion is sane from GCC, but I haven't properly looked into it.)

So personally I'm not concerned about the fact that shrink wrapping doesn't make a difference in my benchmarks; I can show that it changes C code we expect it to change... I'm not so sure about the lack of differences with SPEC though.


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https://reviews.llvm.org/D62190