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

[Matt Arsenault via llvm-commits]

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+; RUN: llc < %s -stop-after=finalize-isel | FileCheck %s --implicit-check-not=FAKE_USE
+;
+; Make sure SelectionDAG does not crash handling fake uses of zero-length arrays
+; and structs. Check also that they are not propagated.
+;
+; Generated from the following source with
+; clang -fextend-lifetimes -S -emit-llvm -O2 -mllvm -stop-after=safe-stack -o test.mir test.cpp
+;
+; int main ()
+; { int array[0]; }
+;
+;
+; CHECK: liveins: $[[IN_REG:[a-zA-Z0-9]+]]
+; CHECK: %[[IN_VREG:[a-zA-Z0-9]+]]:gr32 = COPY $[[IN_REG]]
+; CHECK: FAKE_USE %[[IN_VREG]]
+
+source_filename = "test.ll"
+target datalayout = "e-m:e-i64:64-f80:128-n8:16:32:64-S128"
+
+define hidden i32 @main([0 x i32] %zero, [1 x i32] %one) local_unnamed_addr {
+entry:
+  notail call void (...) @bar([0 x i32] %zero)
+  notail call void (...) @baz([1 x i32] %one)
+  notail call void (...) @llvm.fake.use([0 x i32] %zero)
+  notail call void (...) @llvm.fake.use([1 x i32] %one)
+  ret i32 0
+}
+
+declare void @bar([0 x i32] %a)
+declare void @baz([1 x i32] %a)
+
+; Function Attrs: nounwind
+declare void @llvm.fake.use(...)
+
+!llvm.module.flags = !{!0, !1}
+!llvm.ident = !{!2}
+
+!0 = !{i32 1, !"wchar_size", i32 2}
+!1 = !{i32 7, !"PIC Level", i32 2}
+!2 = !{!"clang version 10.0.0"}
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arsenm wrote:

Missing newline at end of file 

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