[ConstantFolding] Add flag to disable call folding (#140270)

Add an optional flag to disable constant-folding for function calls.
This applies to both intrinsics and libcalls.

This is not necessary in most cases, so is disabled by default, but in
cases that require bit-exact precision between the result from
constant-folding and run-time execution, having this flag can be useful,
and may help with debugging. Cases where mismatches can occur include
GPU execution vs host-side folding, cross-compilation scenarios, or
compilation vs execution environments with different math library
versions.

This applies only to calls, rather than all FP arithmetic. Methods such
as fast-math-flags can be used to limit reassociation, fma-fusion etc,
and basic arithmetic operations are precisely defined in IEEE 754.
However, other math operations such as sqrt, sin, pow etc. represented
by either libcalls or intrinsics are less well defined, and may vary
more between different architectures/library implementations.

As this option is not intended for most common use-cases, this patch
takes the more conservative approach of disabling constant-folding even
for operations like fmax, copysign, fabs etc. in order to keep the
implementation simple, rather than sprinkling checks for this flag
throughout.

The use-cases for this option are similar to StrictFP, but it is only
limited to FP call folding, rather than all FP operations, as it is
about precise arithmetic results, rather than FP environment behaviours.
It also can be used to when linking .bc files compiled with different
StrictFP settings with llvm-link.

0 files changed, 0 insertions, 0 deletions