[ConstantFold] Use ConstantFoldLoadFromUniformValue() in more places

In particular, this also preserves undef when loading from padding,
rather than converting it to zero through a different codepath.

This is the remaining part of D115924.

Author: nikic

clang/test/CodeGen/aapcs-align.cpp

@@ -134,8 +134,8 @@ void g6() {
   f6m(1, 2, 3, 4, 5, s);
 }
 // CHECK: define{{.*}} void @g6
-// CHECK: call void @f6(i32 1, [4 x i32] [i32 6, i32 7, i32 0, i32 0])
-// CHECK: call void @f6m(i32 1, i32 2, i32 3, i32 4, i32 5, [4 x i32] [i32 6, i32 7, i32 0, i32 0])
+// CHECK: call void @f6(i32 1, [4 x i32] [i32 6, i32 7, i32 0, i32 undef])
+// CHECK: call void @f6m(i32 1, i32 2, i32 3, i32 4, i32 5, [4 x i32] [i32 6, i32 7, i32 0, i32 undef])
 // CHECK: declare void @f6(i32, [4 x i32])
 // CHECK: declare void @f6m(i32, i32, i32, i32, i32, [4 x i32])
 }

llvm/lib/Analysis/ConstantFolding.cpp

@@ -106,11 +106,8 @@ Constant *FoldBitCast(Constant *C, Type *DestTy, const DataLayout &DL) {
          "Invalid constantexpr bitcast!");
 
   // Catch the obvious splat cases.
-  if (C->isNullValue() && !DestTy->isX86_MMXTy() && !DestTy->isX86_AMXTy())
-    return Constant::getNullValue(DestTy);
-  if (C->isAllOnesValue() && !DestTy->isX86_MMXTy() && !DestTy->isX86_AMXTy() &&
-      !DestTy->isPtrOrPtrVectorTy()) // Don't get ones for ptr types!
-    return Constant::getAllOnesValue(DestTy);
+  if (Constant *Res = ConstantFoldLoadFromUniformValue(C, DestTy))
+    return Res;
 
   if (auto *VTy = dyn_cast<VectorType>(C->getType())) {
     // Handle a vector->scalar integer/fp cast.
@@ -362,16 +359,8 @@ Constant *llvm::ConstantFoldLoadThroughBitcast(Constant *C, Type *DestTy,
 
     // Catch the obvious splat cases (since all-zeros can coerce non-integral
     // pointers legally).
-    if (C->isNullValue() && !DestTy->isX86_MMXTy() && !DestTy->isX86_AMXTy())
-      return Constant::getNullValue(DestTy);
-    if (C->isAllOnesValue() &&
-        (DestTy->isIntegerTy() || DestTy->isFloatingPointTy() ||
-         DestTy->isVectorTy()) &&
-        !DestTy->isX86_AMXTy() && !DestTy->isX86_MMXTy() &&
-        !DestTy->isPtrOrPtrVectorTy())
-      // Get ones when the input is trivial, but
-      // only for supported types inside getAllOnesValue.
-      return Constant::getAllOnesValue(DestTy);
+    if (Constant *Res = ConstantFoldLoadFromUniformValue(C, DestTy))
+      return Res;
 
     // If the type sizes are the same and a cast is legal, just directly
     // cast the constant.

llvm/test/Transforms/InstSimplify/ConstProp/loads.ll

@@ -320,7 +320,7 @@ define i8 @load_neg_one_at_unknown_offset() {
 
 define i32 @load_padding() {
 ; CHECK-LABEL: @load_padding(
-; CHECK-NEXT:    ret i32 0
+; CHECK-NEXT:    ret i32 undef
 ;
   %v = load i32, i32* getelementptr (i32, i32* bitcast ({ i32, [4 x i8] }* @g_with_padding to i32*), i64 1)
   ret i32 %v