[llvm] [LoopUnroll] Fix assert fail on zeroed branch weights (PR #165938)

[Lei Wang via llvm-commits]

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@@ -0,0 +1,30 @@
+; Check that zeroed branch weights do not crash or otherwise break basic
+; LoopUnroll behavior when it tries to compute a probability from them.
+
+; RUN: opt < %s -S -unroll-count=2 -passes='loop-unroll' 2>&1 | FileCheck %s
+
+define void @test() {
+entry:
+  br label %loop
+
+loop:
+  br i1 false, label %end, label %loop, !prof !0
+
+end:
+  ret void
+}
+
+!0 = !{!"branch_weights", i32 0, i32 0}
+
+; CHECK: define void @test() {
+; CHECK: entry:
+; CHECK:   br label %loop
+; CHECK: loop:
+; CHECK:   br i1 false, label %end, label %loop.1, !prof !0
+; CHECK: loop.1:
+; CHECK:   br i1 false, label %end, label %loop, !prof !0, !llvm.loop !1
+; CHECK-NOT: loop.2
+; CHECK: end:
+; CHECK:   ret void
+; CHECK: }
+; CHECK: !0 = !{!"branch_weights", i32 0, i32 0}
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wlei-llvm wrote:

I guess we alternatively can avoid to create all zero branch_weights metadata by setting the `ElideAllZero` parameter in `setBranchWeights `, in this case we can save some metadata memory. 

https://github.com/llvm/llvm-project/pull/165938