[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 @@
+; Suppress redundant loads feeding into fake uses.
+; RUN: llc -filetype=asm -o - %s --mtriple=x86_64-unknown-unknown | FileCheck %s
+; Windows ABI works differently, there's no offset.
+;
+; Look for the spill
+; CHECK:      movq %r{{[a-z]+,}} -{{[0-9]+\(%rsp\)}}
+; CHECK-NOT:  movq -{{[0-9]+\(%rsp\)}}, %r{{[a-z]+}}
+
+define dso_local i32 @f(ptr %p) local_unnamed_addr {
+entry:
+  call void asm sideeffect "", "~{rax},~{rbx},~{rcx},~{rdx},~{rsi},~{rdi},~{rbp},~{r8},~{r9},~{r10},~{r11},~{r12},~{r13},~{r14},~{r15},~{dirflag},~{fpsr},~{flags}"() #1, !srcloc !2
+  notail call void (...) @llvm.fake.use(ptr %p)
+  ret i32 4
+}
+
+declare void @llvm.fake.use(...) #1
+
+!llvm.module.flags = !{!0}
+!llvm.ident = !{!1}
+
+!0 = !{i32 1, !"wchar_size", i32 4}
+!1 = !{!"clang version 9.0.0"}
+!2 = !{i32 -2147471544}
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arsenm wrote:

Don't need the metadata 

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