[llvm] r350753 - [CodeGen] Ignore return sext/zext attributes of unused results for tail calls

[Francis Visoiu Mistrih via llvm-commits]

Author: thegameg
Date: Wed Jan  9 11:46:15 2019
New Revision: 350753

URL: http://llvm.org/viewvc/llvm-project?rev=350753&view=rev
Log:
[CodeGen] Ignore return sext/zext attributes of unused results for tail calls

If the caller's return type does not have a zeroext attribute but the
callee does a tail call zeroext, we won't consider the tail call during
CodeGenPrepare because the attributes don't match.

However, if the result of the tail call has no uses, it makes sense to
drop the sext/zext attributes.

Differential Revision: https://reviews.llvm.org/D56486

Added:
    llvm/trunk/test/CodeGen/AArch64/tail-call-unused-zext.ll
Modified:
    llvm/trunk/lib/CodeGen/Analysis.cpp

Modified: llvm/trunk/lib/CodeGen/Analysis.cpp
URL: http://llvm.org/viewvc/llvm-project/llvm/trunk/lib/CodeGen/Analysis.cpp?rev=350753&r1=350752&r2=350753&view=diff
==============================================================================
--- llvm/trunk/lib/CodeGen/Analysis.cpp (original)
+++ llvm/trunk/lib/CodeGen/Analysis.cpp Wed Jan  9 11:46:15 2019
@@ -546,6 +546,21 @@ bool llvm::attributesPermitTailCall(cons
     CalleeAttrs.removeAttribute(Attribute::SExt);
   }
 
+  // Drop sext and zext return attributes if the result is not used.
+  // This enables tail calls for code like:
+  //
+  // define void @caller() {
+  // entry:
+  //   %unused_result = tail call zeroext i1 @callee()
+  //   br label %retlabel
+  // retlabel:
+  //   ret void
+  // }
+  if (I->use_empty()) {
+    CalleeAttrs.removeAttribute(Attribute::SExt);
+    CalleeAttrs.removeAttribute(Attribute::ZExt);
+  }
+
   // If they're still different, there's some facet we don't understand
   // (currently only "inreg", but in future who knows). It may be OK but the
   // only safe option is to reject the tail call.

Added: llvm/trunk/test/CodeGen/AArch64/tail-call-unused-zext.ll
URL: http://llvm.org/viewvc/llvm-project/llvm/trunk/test/CodeGen/AArch64/tail-call-unused-zext.ll?rev=350753&view=auto
==============================================================================
--- llvm/trunk/test/CodeGen/AArch64/tail-call-unused-zext.ll (added)
+++ llvm/trunk/test/CodeGen/AArch64/tail-call-unused-zext.ll Wed Jan  9 11:46:15 2019
@@ -0,0 +1,36 @@
+; RUN: llc -mtriple=arm64--- -stop-after=expand-isel-pseudos -o - %s | FileCheck %s
+
+; Check that we ignore the zeroext attribute on the return type of the tail
+; call, since the return value is unused. This happens during CodeGenPrepare in
+; dupRetToEnableTailCallOpts, which calls attributesPermitTailCall to check if
+; the attributes of the caller and the callee match.
+
+declare zeroext i1 @zcallee()
+define void @zcaller() {
+; CHECK-LABEL: name: zcaller
+entry:
+  br i1 undef, label %calllabel, label %retlabel
+calllabel:
+; CHECK: bb.1.calllabel:
+; CHECK-NOT: BL @zcallee
+; CHECK-NEXT: TCRETURNdi @zcallee
+  %unused_result = tail call zeroext i1 @zcallee()
+  br label %retlabel
+retlabel:
+  ret void
+}
+
+declare signext i1 @scallee()
+define void @scaller() {
+; CHECK-LABEL: name: scaller
+entry:
+  br i1 undef, label %calllabel, label %retlabel
+calllabel:
+; CHECK: bb.1.calllabel:
+; CHECK-NOT: BL @scallee
+; CHECK-NEXT: TCRETURNdi @scallee
+  %unused_result = tail call signext i1 @scallee()
+  br label %retlabel
+retlabel:
+  ret void
+}