[PATCH] D128591: Transforms: refactor pow(x, n) expansion where n is a constant integer value

[eaeltsin via Phabricator via llvm-commits]

eaeltsin added a comment.

Hi,

This patch introduces the difference between new inlined and library versions of `std::pow`,  at least on `x86_64`. For large exponents, the difference is large. Also, when the difference accumulates, it makes a lot of existing tests that compare against golden values to go far beyond the meaningful margin.

Similarly, it introduces the difference between `pow` of constant and `pow` of non-constant with the same value.

I think such differences make related debugging a lot more complex. What do you think?

Here are the tests:

  [[clang::optnone]] double test_pow(double x, double y) {
    return std::pow(x, y);
  }
  
  // PASS - compiler evaluates std::pow(const, const) similar to the library?
  TEST(Test, Pow_const) {
    double x = 8.804009981602594;
    double y = 10.0;
    double t = test_pow(x, y);
    double z = std::pow(x, y);
    EXPECT_TRUE(t - z < 1e-6 && z -t < 1e-6);
  }
  
  [[clang::optnone]] double init(const char* buf) {
    double x;
    memcpy(&x, buf, sizeof(double));
    return x;
  }
  
  // FAIL - https://reviews.llvm.org/D128591 introduces difference.
  TEST(Test, Pow_nonconst) {
    double x = init("\x0a\x48\x41\x32\xa7\x9b\x21\x40");  // 8.804009981602594
    double y = 10.0;
    double t = test_pow(x, y);
    double z = std::pow(x, y);
    EXPECT_TRUE(t - z < 1e-6 && z -t < 1e-6);
  }


Repository:
  rG LLVM Github Monorepo

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  https://reviews.llvm.org/D128591/new/

https://reviews.llvm.org/D128591