[llvm] [SimplifyCFG][PGO] Add missing overflow check to ConstantFoldTerminator (PR #178964)

[Mitchel Dickerson via llvm-commits]

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@@ -0,0 +1,27 @@
+; RUN: opt < %s -passes=simplifycfg -S | FileCheck %s
+
+; Ensure branch weight does not overflow when folding block 3 into block 0.
+; This IR test is reduced from an optimization that occurs during the Jump
+; Threading pass, which creates a branch prediction of 0% and 100%. This is
+; reduced in SimplifyCFG into the IR shown in the
+; switch-branch-weight-overflow.ll test.
+
+define void @foo(ptr %Overflow) {
+  %1 = extractvalue { i32, i1 } zeroinitializer, 0
+  %2 = icmp eq i32 %1, 0
+  br i1 %2, label %6, label %3
+
+3:                                                ; preds = %0
+  %4 = icmp eq i32 %1, 1
+  br i1 %4, label %5, label %6, !prof !0
+
+5:                                                ; preds = %3
+  store i32 0, ptr %Overflow, align 4
+  br label %6
+
+6:                                                ; preds = %5, %3, %0
+  ret void
+}
+
+; CHECK: branch_weights{{.*}} 0, {{.*}} -2147483648
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mauve-stinger wrote:

OK, sounds good. I will change the `CHECK` line to be explicit then. Thank you for the explanation!

https://github.com/llvm/llvm-project/pull/178964