[ConstantFolding] Fold constrained arithmetic intrinsics

Constfold constrained variants of operations fadd, fsub, fmul, fdiv,
frem, fma and fmuladd.

The change also sets up some means to support for removal of unused
constrained intrinsics. They are declared as accessing memory to model
interaction with floating point environment, so they were not removed,
as they have side effect. Now constrained intrinsics that have
"fpexcept.ignore" as exception behavior are removed if they have no uses.
As for intrinsics that have exception behavior other than "fpexcept.ignore",
they can be removed if it is known that they do not raise floating point
exceptions. It happens when doing constant folding, attributes of such
intrinsic are changed so that the intrinsic is not claimed as accessing
memory.

Differential Revision: https://reviews.llvm.org/D102673

1 files changed, 10 insertions, 0 deletions

diff --git a/llvm/lib/Transforms/Utils/Local.cpp b/llvm/lib/Transforms/Utils/Local.cpp
index d5e301b..d95c053 100644
--- a/llvm/lib/Transforms/Utils/Local.cpp
+++ b/llvm/lib/Transforms/Utils/Local.cpp
@@ -491,6 +491,16 @@ bool llvm::wouldInstructionBeTriviallyDead(Instruction *I,
     if (isMathLibCallNoop(Call, TLI))
       return true;
 
+  // To express possible interaction with floating point environment constrained
+  // intrinsics are described as if they access memory. So they look like having
+  // side effect but actually do not have it unless they raise floating point
+  // exception. If FP exceptions are ignored, the intrinsic may be deleted.
+  if (auto *CI = dyn_cast<ConstrainedFPIntrinsic>(I)) {
+    Optional<fp::ExceptionBehavior> EB = CI->getExceptionBehavior();
+    if (!EB || *EB == fp::ExceptionBehavior::ebIgnore)
+      return true;
+  }
+
   return false;
 }