[llvm] r228885 - DeadArgElim: aggregate Return assessment properly.

[Tim Northover]

Author: tnorthover
Date: Wed Feb 11 17:13:11 2015
New Revision: 228885

URL: http://llvm.org/viewvc/llvm-project?rev=228885&view=rev
Log:
DeadArgElim: aggregate Return assessment properly.

I mistakenly thought the liveness of each "RetVal(F, i)" depended only on F. It
actually depends on the index too, which means we need to be careful about how
the results are combined before return. In particular if a single Use returns
Live, that counts for the entire object, at the granularity we're considering.

Modified:
    llvm/trunk/lib/Transforms/IPO/DeadArgumentElimination.cpp
    llvm/trunk/test/Transforms/DeadArgElim/aggregates.ll

Modified: llvm/trunk/lib/Transforms/IPO/DeadArgumentElimination.cpp
URL: http://llvm.org/viewvc/llvm-project/llvm/trunk/lib/Transforms/IPO/DeadArgumentElimination.cpp?rev=228885&r1=228884&r2=228885&view=diff
==============================================================================
--- llvm/trunk/lib/Transforms/IPO/DeadArgumentElimination.cpp (original)
+++ llvm/trunk/lib/Transforms/IPO/DeadArgumentElimination.cpp Wed Feb 11 17:13:11 2015
@@ -449,12 +449,15 @@ DAE::Liveness DAE::SurveyUse(const Use *
         // We might be live, depending on the liveness of Use.
         return MarkIfNotLive(Use, MaybeLiveUses);
       } else {
-        DAE::Liveness Result;
+        DAE::Liveness Result = MaybeLive;
         for (unsigned i = 0; i < NumRetVals(F); ++i) {
           RetOrArg Use = CreateRet(F, i);
-          // We might be live, depending on the liveness of Use. All Results
-          // should be the same since they depend only on F.
-          Result = MarkIfNotLive(Use, MaybeLiveUses);
+          // We might be live, depending on the liveness of Use. If any
+          // sub-value is live, then the entire value is considered live. This
+          // is a conservative choice, and better tracking is possible.
+          DAE::Liveness SubResult = MarkIfNotLive(Use, MaybeLiveUses);
+          if (Result != Live)
+            Result = SubResult;
         }
         return Result;
       }

Modified: llvm/trunk/test/Transforms/DeadArgElim/aggregates.ll
URL: http://llvm.org/viewvc/llvm-project/llvm/trunk/test/Transforms/DeadArgElim/aggregates.ll?rev=228885&r1=228884&r2=228885&view=diff
==============================================================================
--- llvm/trunk/test/Transforms/DeadArgElim/aggregates.ll (original)
+++ llvm/trunk/test/Transforms/DeadArgElim/aggregates.ll Wed Feb 11 17:13:11 2015
@@ -130,3 +130,33 @@ define i32 @test_ret_applies_to_all() {
   %ret = extractvalue {i32, i32} %val, 1
   ret i32 %ret
 }
+
+; Case 6: When considering @mid, the return instruciton has sub-value 0
+; unconditionally live, but 1 only conditionally live. Since at that level we're
+; applying the results to the whole of %res, this means %res is live and cannot
+; be reduced. There is scope for further optimisation here (though not visible
+; in this test-case).
+
+; CHECK-LABEL: define internal { i8*, i32 } @inner()
+
+define internal {i8*, i32} @mid() {
+  %res = call {i8*, i32} @inner()
+  %intval = extractvalue {i8*, i32} %res, 1
+  %tst = icmp eq i32 %intval, 42
+  br i1 %tst, label %true, label %true
+
+true:
+  ret {i8*, i32} %res
+}
+
+define internal {i8*, i32} @inner() {
+  ret {i8*, i32} {i8* null, i32 42}
+}
+
+define internal i8 @outer() {
+  %res = call {i8*, i32} @mid()
+  %resptr = extractvalue {i8*, i32} %res, 0
+
+  %val = load i8* %resptr
+  ret i8 %val
+}
\ No newline at end of file