[llvm-dev] FENV_ACCESS and floating point LibFunc calls

Kaylor, Andrew via llvm-dev
2017-05-10T23:17:19Z

Hi all,

Background
I've been working on adding the necessary support to LLVM for clang to be
able to support the STDC FENV_ACCESS pragma, which basically allows users to
modify rounding mode at runtime and depend on the value of floating-point status
flags or to unmask floating point exceptions without unexpected side effects.
I've committed an initial patch (r293226) that adds constrained intrinsics
for the basic FP operations, and I have a patch up for review now
(https://reviews.llvm.org/D32319) that adds constrained versions of a number of
libm-like FP intrinsics.

Current problem
Now I'm trying to make sure I have a good solution for the way in which the
optimizer handles recognized calls to libm functions (sqrt, pow, cos, sin,
etc.).  Basically, I need to prevent all passes from making any modifications to
these calls that would make assumptions about rounding mode or improperly affect
the FP status flags (either suppressing flags that should be present or setting
flags that should not be set).  For instance, there are circumstances in which
the optimizer will constant fold a call to one of these functions if the value
of the arguments are known at compile time, but this constant folding generally
assumes the default rounding mode and if the library call would have set a
status flag, I need the flag to be set.

Question
My question is, can/should I just rely on the front end setting the
"nobuiltin" attribute for the call site in any location where the FP
behavior needs to be restricted?

Ideally, I would like to be able to conditionally enable optimizations like
constant folding if I am able to prove that the rounding mode, though dynamic,
is known for the callsite at compile time (the constrained intrinsics have a
mechanism to enable this), but at the moment I am more concerned about
correctness and would be willing to sacrifice optimizations to get correct
behavior.  Long term, I was thinking that maybe I could do something like attach
metadata to indicate rounding mode and exception behavior when they were known.

Is there a better way to do this?

Thanks,
Andy

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