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

Mon Oct 5 08:37:50 PDT 2020

mibintc added inline comments.

================
Comment at: clang/include/clang/Basic/LangOptions.h:410-411
     setRoundingMode(LO.getFPRoundingMode());
+    // FENV access is true if rounding math is enabled.
+    setAllowFEnvAccess((getRoundingMode() == llvm::RoundingMode::Dynamic));
     setFPExceptionMode(LO.getFPExceptionMode());
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sepavloff wrote:
> 
> I don't think this deduction is correct.
> 
> `AllowFEnvAccess` implies that user can access FP environment in a way unknown to the compiler, for example by calling external functions or by using inline assembler. So if `AllowFEnvAccess` is set, the compiler must assume that rounding mode is dynamic, as user can set it to any value unknown to the compiler. Similarly, `FPExceptionMode` must be set to strict as user may read/write exception bits in any way or may associate them with traps. These deductions are valid and compiler must apply them.
> 
> The reverse is not valid. `AllowFEnvAccess` is a very coarse instrument to manipulate FP environment, it prevents from many optimization techniques. In contrast, setting `RoundingMode` to dynamic means only that rounding mode may be different from the default value. If a user changes rounding mode by calls to external functions like `fesetmode` or by using some intrinsic, the compiler still can, for example, reorder FP operations, if `FPExceptionMode` is `ignore`. Impact of performance in this case can be minimized.
> 
> 
Thanks @sepavloff I'll make the change you recommend

Repository:
  rG LLVM Github Monorepo

CHANGES SINCE LAST ACTION
  https://reviews.llvm.org/D87528/new/

https://reviews.llvm.org/D87528