sepavloff 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());
----------------
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.
Repository:
rG LLVM Github Monorepo
CHANGES SINCE LAST ACTION
https://reviews.llvm.org/D87528/new/
https://reviews.llvm.org/D87528
_______________________________________________
cfe-commits mailing list
[email protected]
https://lists.llvm.org/cgi-bin/mailman/listinfo/cfe-commits
