[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, 7 insertions, 4 deletions

diff --git a/llvm/lib/Analysis/ConstantFolding.cpp b/llvm/lib/Analysis/ConstantFolding.cpp
index fab73c2..2f164e8 100644
--- a/llvm/lib/Analysis/ConstantFolding.cpp
+++ b/llvm/lib/Analysis/ConstantFolding.cpp
@@ -1435,7 +1435,7 @@ Constant *llvm::ConstantFoldCastOperand(unsigned Opcode, Constant *C,
                                             /*IsSigned=*/false);
       }
     }
-    return ConstantExpr::getCast(Opcode, C, DestTy);
+    break;
   case Instruction::IntToPtr:
     // If the input is a ptrtoint, turn the pair into a ptr to ptr bitcast if
     // the int size is >= the ptr size and the address spaces are the same.
@@ -1454,8 +1454,7 @@ Constant *llvm::ConstantFoldCastOperand(unsigned Opcode, Constant *C,
         }
       }
     }
-
-    return ConstantExpr::getCast(Opcode, C, DestTy);
+    break;
   case Instruction::Trunc:
   case Instruction::ZExt:
   case Instruction::SExt:
@@ -1466,10 +1465,14 @@ Constant *llvm::ConstantFoldCastOperand(unsigned Opcode, Constant *C,
   case Instruction::FPToUI:
   case Instruction::FPToSI:
   case Instruction::AddrSpaceCast:
-      return ConstantExpr::getCast(Opcode, C, DestTy);
+    break;
   case Instruction::BitCast:
     return FoldBitCast(C, DestTy, DL);
   }
+
+  if (ConstantExpr::isDesirableCastOp(Opcode))
+    return ConstantExpr::getCast(Opcode, C, DestTy);
+  return ConstantFoldCastInstruction(Opcode, C, DestTy);
 }
 
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