[llvm] [ExtendLifetimes] Implement llvm.fake.use to extend variable lifetimes (PR #86149)

[Matt Arsenault via llvm-commits]

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@@ -0,0 +1,23 @@
+; RUN: llc < %s -stop-after=finalize-isel -O2 - | FileCheck %s --implicit-check-not FAKE_USE
+; Fake uses following tail calls should be pulled in front
+; of the TCRETURN instruction. Fake uses using something defined by
+; the tail call or after it should be suppressed.
+
+; CHECK: body:
+; CHECK: bb.0.{{.*}}:
+; CHECK: %0:{{.*}}= COPY
+; CHECK: FAKE_USE %0
+; CHECK: TCRETURN
+
+define void @bar(i32 %v) {
+entry:
+  %call = tail call i32 @_Z3fooi(i32 %v)
+  %mul = mul nsw i32 %call, 3
+  notail call void (...) @llvm.fake.use(i32 %mul)
+  notail call void (...) @llvm.fake.use(i32 %call)
+  notail call void (...) @llvm.fake.use(i32 %v)
+  ret void
+}
+
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arsenm wrote:

Test a case where the value produced by the call isn't used by the fake use, and is the return value? 

https://github.com/llvm/llvm-project/pull/86149