[PATCH] D87528: Enable '#pragma STDC FENV_ACCESS' in frontend cf. D69272 - Work in Progress

[Serge Pavlov via Phabricator via cfe-commits]

sepavloff added inline comments.


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Comment at: clang/test/CodeGen/fp-floatcontrol-pragma.cpp:154
+  if (i<0)
+  return 1.0 + 2.0;
+  // Check that floating point constant folding doesn't occur if
----------------
In this particular case we know for sure that the result does not depend on rounding mode and no FP exceptions occurs, so it is safe to constfold the expression.

Expressions like `1.0 / 0.0` or `1.0F + 0x0.000001p0F` indeed may require execution in runtime, depending on the required exception handling. I would propose to connect their const-evaluability with `FPExceptionMode` and set the latter to `strict` whenever `AllowFEnvAccess` is set to `true`. 


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Comment at: clang/test/CodeGen/pragma-fenv_access.c:9
+// CHECK-LABEL: @func_01
+// CHECK: call float @llvm.experimental.constrained.fadd.f32(float {{.*}}, float {{.*}}, metadata !"round.tonearest", metadata !"fpexcept.strict")
+
----------------
Shall the rounding mode be `dynamic` rather than `tonearest`? If `#pragma STDC FENV_ACCESS ON` is specified, it means FP environment may be changed in arbitrary way and we cannot expect any particular rounding mode. Probably the environment was changed in the caller of `func_01`.


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  https://reviews.llvm.org/D87528/new/

https://reviews.llvm.org/D87528