[LLVM][ConstantFold] Undefined values are not constant (#130713)

llvm.is.constant (and therefore Clang's __builtin_constant_p()) need to
report undefined values as non-constant or future DCE choices end up
making no sense. This was encountered while building the Linux kernel
which uses __builtin_constant_p() while trying to evaluate if it is safe
to use a compile-time constant resolution for string lengths or if it
must kick over to a full runtime call to strlen(). Obviously an
undefined variable cannot be known at compile-time, so
__builtin_constant_p() needs to return false. This change will also mean
that Clang will match GCC's behavior under the same conditions.

Fixes #130649

Author: kees

llvm/lib/IR/Constants.cpp

@@ -841,6 +841,8 @@ Constant *Constant::mergeUndefsWith(Constant *C, Constant *Other) {
 }
 
 bool Constant::isManifestConstant() const {
+  if (isa<UndefValue>(this))
+    return false;
   if (isa<ConstantData>(this))
     return true;
   if (isa<ConstantAggregate>(this) || isa<ConstantExpr>(this)) {

llvm/test/Transforms/LowerConstantIntrinsics/constant-intrinsics.ll

@@ -120,3 +120,21 @@ define i1 @global_array() {
   %1 = call i1 @llvm.is.constant.i64(i64 ptrtoint (ptr @real_mode_blob_end to i64))
   ret i1 %1
 }
+
+;; Ensure that is.constant of undef gets lowered reasonably to "false" in
+;; optimized codegen: specifically that the "true" branch is eliminated.
+;; CHECK-NOT: tail call i32 @subfun_1()
+;; CHECK:     tail call i32 @subfun_2()
+;; CHECK-NOT: tail call i32 @subfun_1()
+define i32 @test_undef_branch() nounwind {
+  %v = call i1 @llvm.is.constant.i32(i32 undef)
+  br i1 %v, label %True, label %False
+
+True:
+  %call1 = tail call i32 @subfun_1()
+  ret i32 %call1
+
+False:
+  %call2 = tail call i32 @subfun_2()
+  ret i32 %call2
+}