[IR] Mark zext/sext constant expressions as undesirable

Introduce isDesirableCastOp() which determines whether IR builder
and constant folding should produce constant expressions for a
given cast type. This mirrors what we do for binary operators.

Mark zext/sext as undesirable, which prevents most creations of such
constant expressions. This is still somewhat incomplete and there
are a few more places that can create zext/sext expressions.

This is part of the work for
https://discourse.llvm.org/t/rfc-remove-most-constant-expressions/63179.

The reason for the odd result in the constantexpr-fneg.c test is
that initially the "a[]" global is created with an [0 x i32] type,
at which point the icmp expression cannot be folded. Later it is
replaced with an [1 x i32] global and the icmp gets folded away.
But at that point we no longer fold the zext.

1 files changed, 6 insertions, 4 deletions

diff --git a/llvm/lib/Transforms/Utils/SCCPSolver.cpp b/llvm/lib/Transforms/Utils/SCCPSolver.cpp
index 8a67fda..4b96b02 100644
--- a/llvm/lib/Transforms/Utils/SCCPSolver.cpp
+++ b/llvm/lib/Transforms/Utils/SCCPSolver.cpp
@@ -1225,10 +1225,12 @@ void SCCPInstVisitor::visitCastInst(CastInst &I) {
 
   if (Constant *OpC = getConstant(OpSt, I.getOperand(0)->getType())) {
     // Fold the constant as we build.
-    Constant *C = ConstantFoldCastOperand(I.getOpcode(), OpC, I.getType(), DL);
-    markConstant(&I, C);
-  } else if (I.getDestTy()->isIntegerTy() &&
-             I.getSrcTy()->isIntOrIntVectorTy()) {
+    if (Constant *C =
+            ConstantFoldCastOperand(I.getOpcode(), OpC, I.getType(), DL))
+      return (void)markConstant(&I, C);
+  }
+
+  if (I.getDestTy()->isIntegerTy() && I.getSrcTy()->isIntOrIntVectorTy()) {
     auto &LV = getValueState(&I);
     ConstantRange OpRange = getConstantRange(OpSt, I.getSrcTy());