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

[Matt Arsenault via llvm-commits]

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@@ -0,0 +1,53 @@
+; RUN: opt -mtriple=x86_64-unknown-unknown -S -codegenprepare <%s -o - | FileCheck %s
+;
+; Ensure return instruction splitting ignores fake uses.
+;
+; IR Generated with clang -O2 -S -emit-llvm -fextend-lifetimes test.cpp
+;
+;// test.cpp
+;extern int bar(int);
+;
+;int foo2(int i)
+;{
+;  --i;
+;  if (i <= 0)
+;    return -1;
+;  return bar(i);
+;}
+
+; ModuleID = 'test.cpp'
+source_filename = "test.cpp"
+target datalayout = "e-m:e-i64:64-f80:128-n8:16:32:64-S128"
+target triple = "x86_64-unknown-unknown"
+
+declare i32 @_Z3bari(i32) local_unnamed_addr
+
+; Function Attrs: nounwind
+declare void @llvm.fake.use(...)
+
+; Function Attrs: nounwind sspstrong uwtable
+define i32 @_Z4foo2i(i32 %i) local_unnamed_addr {
+entry:
+  %dec = add nsw i32 %i, -1
+  %cmp = icmp slt i32 %i, 2
+  br i1 %cmp, label %cleanup, label %if.end
+
+if.end:                                           ; preds = %entry
+  %call = tail call i32 @_Z3bari(i32 %dec)
+; CHECK: ret i32 %call
+  br label %cleanup
+
+cleanup:                                          ; preds = %entry, %if.end
+; CHECK: cleanup:
+  %retval.0 = phi i32 [ %call, %if.end ], [ -1, %entry ]
+  tail call void (...) @llvm.fake.use(i32 %dec)
+; CHECK: ret i32 -1
+  ret i32 %retval.0
+}
+
+!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 7.0.0"}
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arsenm wrote:

Don't need the metadata 

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