[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, 22 insertions, 0 deletions

diff --git a/llvm/lib/IR/Constants.cpp b/llvm/lib/IR/Constants.cpp
index 6589bd3..ba18da9 100644
--- a/llvm/lib/IR/Constants.cpp
+++ b/llvm/lib/IR/Constants.cpp
@@ -2331,6 +2331,28 @@ bool ConstantExpr::isSupportedBinOp(unsigned Opcode) {
   }
 }
 
+bool ConstantExpr::isDesirableCastOp(unsigned Opcode) {
+  switch (Opcode) {
+  case Instruction::ZExt:
+  case Instruction::SExt:
+    return false;
+  case Instruction::Trunc:
+  case Instruction::FPTrunc:
+  case Instruction::FPExt:
+  case Instruction::UIToFP:
+  case Instruction::SIToFP:
+  case Instruction::FPToUI:
+  case Instruction::FPToSI:
+  case Instruction::PtrToInt:
+  case Instruction::IntToPtr:
+  case Instruction::BitCast:
+  case Instruction::AddrSpaceCast:
+    return true;
+  default:
+    llvm_unreachable("Argument must be cast opcode");
+  }
+}
+
 Constant *ConstantExpr::getSizeOf(Type* Ty) {
   // sizeof is implemented as: (i64) gep (Ty*)null, 1
   // Note that a non-inbounds gep is used, as null isn't within any object.