[PATCH] D66096: [CodeGen/Analysis] Intrinsic llvm.assume should not block tail call optimization

[Guozhi Wei via Phabricator via llvm-commits]

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In function Analysis.cpp:isInTailCallPosition, instructions between call and ret are checked to see if they block tail call optimization. If an instruction is an intrinsic call, only llvm.lifetime_end is allowed and other intrinsic functions block tail call. When compiling tcmalloc, we found llvm.assume between a hot function call and ret, it blocks the optimization. But llvm.assume doesn't generate instructions, it should not block tail call.


Repository:
  rL LLVM

https://reviews.llvm.org/D66096

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


Index: test/CodeGen/X86/tailcall-assume.ll
===================================================================
--- test/CodeGen/X86/tailcall-assume.ll
+++ test/CodeGen/X86/tailcall-assume.ll
@@ -0,0 +1,15 @@
+; RUN: llc -mtriple=x86_64-linux < %s | FileCheck %s
+
+; Intrinsic call to @llvm.assume should not prevent tail call optimization.
+; CHECK-LABEL: foo:
+; CHECK:       jmp bar # TAILCALL
+define i8* @foo() {
+  %1 = tail call i8* @bar()
+  %2 = icmp ne i8* %1, null
+  tail call void @llvm.assume(i1 %2)
+  ret i8* %1
+}
+
+declare i8* @bar()
+declare void @llvm.assume(i1)
+
Index: lib/CodeGen/Analysis.cpp
===================================================================
--- lib/CodeGen/Analysis.cpp
+++ lib/CodeGen/Analysis.cpp
@@ -538,7 +538,8 @@
         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)
+        if (II->getIntrinsicID() == Intrinsic::lifetime_end ||
+            II->getIntrinsicID() == Intrinsic::assume)
           continue;
       if (BBI->mayHaveSideEffects() || BBI->mayReadFromMemory() ||
           !isSafeToSpeculativelyExecute(&*BBI))


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