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

[Kees Cook via llvm-commits]

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@@ -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()
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kees wrote:

Based on my understanding of this CHECK pattern at the top of the file (where I copied this test style from) the second CHECK-NOT is to make sure there is no re-ordering happening: i.e. it must be fully missing, not just present later.

https://github.com/llvm/llvm-project/pull/130713