DeadArgElim: assess uses of entire return value aggregate.

Previously, a non-extractvalue use of an aggregate return value meant
the entire return was considered live (the algorithm gave up
entirely). This was correct, but conservative. It's better to actually
look at that Use, making the analysis results apply to all sub-values
under consideration.

E.g.

  %val = call { i32, i32 } @whatever()
  [...]
  ret { i32, i32 } %val

The return is using the entire aggregate (sub-values 0 and 1). We can
still simplify @whatever if we can prove that this return is itself
unused.

Also unifies the logic slightly between aggregate and non-aggregate
cases..

llvm-svn: 228558

1 files changed, 26 insertions, 26 deletions

diff --git a/llvm/lib/Transforms/IPO/DeadArgumentElimination.cpp b/llvm/lib/Transforms/IPO/DeadArgumentElimination.cpp
index 4045c09aaa2..365555b78c5 100644
--- a/llvm/lib/Transforms/IPO/DeadArgumentElimination.cpp
+++ b/llvm/lib/Transforms/IPO/DeadArgumentElimination.cpp
@@ -541,7 +541,6 @@ void DAE::SurveyFunction(const Function &F) {
   // Keep track of the number of live retvals, so we can skip checks once all
   // of them turn out to be live.
   unsigned NumLiveRetVals = 0;
-  Type *STy = dyn_cast<StructType>(F.getReturnType());
   // Loop all uses of the function.
   for (const Use &U : F.uses()) {
     // If the function is PASSED IN as an argument, its address has been
@@ -563,34 +562,35 @@ void DAE::SurveyFunction(const Function &F) {
 
     // Now, check how our return value(s) is/are used in this caller. Don't
     // bother checking return values if all of them are live already.
-    if (NumLiveRetVals != RetCount) {
-      if (STy) {
-        // Check all uses of the return value.
-        for (const User *U : TheCall->users()) {
-          const ExtractValueInst *Ext = dyn_cast<ExtractValueInst>(U);
-          if (Ext && Ext->hasIndices()) {
-            // This use uses a part of our return value, survey the uses of
-            // that part and store the results for this index only.
-            unsigned Idx = *Ext->idx_begin();
-            if (RetValLiveness[Idx] != Live) {
-              RetValLiveness[Idx] = SurveyUses(Ext, MaybeLiveRetUses[Idx]);
-              if (RetValLiveness[Idx] == Live)
-                NumLiveRetVals++;
-            }
-          } else {
-            // Used by something else than extractvalue. Mark all return
-            // values as live.
-            for (unsigned i = 0; i != RetCount; ++i )
-              RetValLiveness[i] = Live;
-            NumLiveRetVals = RetCount;
-            break;
-          }
+    if (NumLiveRetVals == RetCount)
+      continue;
+
+    // Check all uses of the return value.
+    for (const Use &U : TheCall->uses()) {
+      if (ExtractValueInst *Ext = dyn_cast<ExtractValueInst>(U.getUser())) {
+        // This use uses a part of our return value, survey the uses of
+        // that part and store the results for this index only.
+        unsigned Idx = *Ext->idx_begin();
+        if (RetValLiveness[Idx] != Live) {
+          RetValLiveness[Idx] = SurveyUses(Ext, MaybeLiveRetUses[Idx]);
+          if (RetValLiveness[Idx] == Live)
+            NumLiveRetVals++;
         }
       } else {
-        // Single return value
-        RetValLiveness[0] = SurveyUses(TheCall, MaybeLiveRetUses[0]);
-        if (RetValLiveness[0] == Live)
+        // Used by something else than extractvalue. Survey, but assume that the
+        // result applies to all sub-values.
+        UseVector MaybeLiveAggregateUses;
+        if (SurveyUse(&U, MaybeLiveAggregateUses) == Live) {
           NumLiveRetVals = RetCount;
+          RetValLiveness.assign(RetCount, Live);
+          break;
+        } else {
+          for (unsigned i = 0; i != RetCount; ++i) {
+            if (RetValLiveness[i] != Live)
+              MaybeLiveRetUses[i].append(MaybeLiveAggregateUses.begin(),
+                                         MaybeLiveAggregateUses.end());
+          }
+        }
       }
     }
   }