[PATCH] D53519: [CodeGen] skip lifetime end marker in isInTailCallPosition

[Robert Lougher via Phabricator via llvm-commits]

rob.lougher created this revision.
rob.lougher added reviewers: majnemer, t.p.northover, hfinkel.

Consider the following program:

  ------------- test.c ---------------
  extern void bar(void);
  
  __attribute__((__noinline__))
  void foo(int *p) {
    *p = 10;
  }
  
  void foobar() {
    int i;
    foo(&i);
    bar();
  }
  ------------------------------------

TailCallElim will correctly identify the call to bar as a tail call (as foo is nocapture). However, it will not be tail call optimized as it fails isInTailCallPosition() due to the lifetime end marker for the variable i:

  define void @foobar() {
  entry:
    %i = alloca i32, align 4
    %0 = bitcast i32* %i to i8*
    call void @llvm.lifetime.start.p0i8(i64 4, i8* nonnull %0)
    call void @foo(i32* nonnull %i)
    tail call void @bar()
    call void @llvm.lifetime.end.p0i8(i64 4, i8* nonnull %0)
    ret void
  }

On x86_64 this generates:

  foobar:
  	pushq	%rax
  	leaq	4(%rsp), %rdi
  	callq	foo
  	callq	bar
  	popq	%rax
  	retq

After this patch we get:

  foobar:
  	pushq	%rax
  	leaq	4(%rsp), %rdi
  	callq	foo
  	popq	%rax
  	jmp	bar                     # TAILCALL


Repository:
  rL LLVM

https://reviews.llvm.org/D53519

Files:
  lib/CodeGen/Analysis.cpp
  test/CodeGen/X86/tailcall-lifetime-end.ll


Index: test/CodeGen/X86/tailcall-lifetime-end.ll
===================================================================
--- test/CodeGen/X86/tailcall-lifetime-end.ll
+++ test/CodeGen/X86/tailcall-lifetime-end.ll
@@ -0,0 +1,27 @@
+; RUN: llc -mtriple=x86_64-unknown-linux-gnu -o - %s | FileCheck %s
+
+; A lifetime end intrinsic should not prevent a call from being tail call
+; optimized.
+
+define void @foobar() {
+; CHECK-LABEL: foobar
+; CHECK: pushq	%rax
+; CHECK: leaq	4(%rsp), %rdi
+; CHECK: callq	foo
+; CHECK: popq	%rax
+; CHECK: jmp	bar
+entry:
+  %i = alloca i32
+  %0 = bitcast i32* %i to i8*
+  call void @llvm.lifetime.start.p0i8(i64 4, i8* nonnull %0)
+  call void @foo(i32* nonnull %i)
+  tail call void @bar()
+  call void @llvm.lifetime.end.p0i8(i64 4, i8* nonnull %0)
+  ret void
+}
+
+declare void @foo(i32* nocapture %p)
+declare void @bar()
+
+declare void @llvm.lifetime.start.p0i8(i64, i8* nocapture)
+declare void @llvm.lifetime.end.p0i8(i64, i8* nocapture)
Index: lib/CodeGen/Analysis.cpp
===================================================================
--- lib/CodeGen/Analysis.cpp
+++ lib/CodeGen/Analysis.cpp
@@ -496,6 +496,10 @@
       // Debug info intrinsics do not get in the way of tail call optimization.
       if (isa<DbgInfoIntrinsic>(BBI))
         continue;
+      // A lifetime end intrinsic should not stop tail call optimization.
+      if (const IntrinsicInst *II = dyn_cast<IntrinsicInst>(BBI))
+        if (II->getIntrinsicID() == Intrinsic::lifetime_end)
+          continue;
       if (BBI->mayHaveSideEffects() || BBI->mayReadFromMemory() ||
           !isSafeToSpeculativelyExecute(&*BBI))
         return false;


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