[PATCH] D69979: clang: Guess at some platform FTZ/DAZ default settings

[Sanjay Patel via Phabricator via cfe-commits]

spatel added a comment.

In D69979#1749198 <https://reviews.llvm.org/D69979#1749198>, @arsenm wrote:

> I just posted the test I wrote here: https://github.com/arsenm/subnormal_test


Thanks. I tried compiling with gcc (can't trust clang since it doesn't honor #pragma STDC FENV_ACCESS ON?). 
And running that on a Ubuntu 17.10 x86-64 system, it's behaving as I would expect. If you compile without -ffast-math,  it asserts:

  With denormals disabled
  a.out: subnormal_test.cpp:33: void fp32_denorm_test(): Assertion `std::fpclassify(subnormal) == FP_SUBNORMAL' failed.

And if you compile with -ffast-math, it asserts:

  In default FP mode
  a.out: subnormal_test.cpp:33: void fp32_denorm_test(): Assertion `std::fpclassify(subnormal) == FP_SUBNORMAL' failed.

This is what I see compiling Craig's csr tester:

  $ cc -O2 csr.c && ./a.out
  1f80
  $ cc -O2 csr.c -ffast-math && ./a.out
  9fc0

FZ is bit 15 (0x8000) and DAZ is bit 6 (0x0040), so they are clear in default (IEEE) mode and set with -ffast-math.


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https://reviews.llvm.org/D69979