[PATCH] D31142: [JumpThread] Do RAUW in case Cond folds to a constant in the CFG

[Xin Tong via Phabricator via llvm-commits]

trentxintong created this revision.

This is done in ProcessThreadableEdges where we compute all the possible
values from all predecessors.


https://reviews.llvm.org/D31142

Files:
  lib/Transforms/Scalar/JumpThreading.cpp
  test/Transforms/JumpThreading/basic.ll


Index: test/Transforms/JumpThreading/basic.ll
===================================================================
--- test/Transforms/JumpThreading/basic.ll
+++ test/Transforms/JumpThreading/basic.ll
@@ -3,6 +3,35 @@
 declare i32 @f1()
 declare i32 @f2()
 declare void @f3()
+declare void @f4(i32)
+
+; Make sure we can do the RAUW for %add...
+;
+; CHECK-LABEL: @rauw_if_possible(
+; CHECK: call void @f4(i32 96) 
+define void @rauw_if_possible(i32 %value) nounwind {
+entry:
+  %cmp = icmp eq i32 %value, 32
+  br i1 %cmp, label %L0, label %L3 
+L0:
+  call i32 @f2()
+  call i32 @f2()
+  %add = add i32 %value, 64
+  switch i32 %add, label %L3 [
+    i32 32, label %L1
+    i32 96, label %L2
+    ]
+
+L1:
+  call void @f3()
+  ret void
+L2:
+  call void @f4(i32 %add)
+  ret void
+L3:
+  call void @f3()
+  ret void
+}
 
 ; Make sure we can fold this branch ... We will not be able to thread it as
 ; L0 is too big to duplicate.
@@ -43,7 +72,6 @@
   call void @f3()
   ret void
 }
-
 define i32 @test1(i1 %cond) {
 ; CHECK-LABEL: @test1(
 
Index: lib/Transforms/Scalar/JumpThreading.cpp
===================================================================
--- lib/Transforms/Scalar/JumpThreading.cpp
+++ lib/Transforms/Scalar/JumpThreading.cpp
@@ -1230,6 +1230,8 @@
 
   BasicBlock *OnlyDest = nullptr;
   BasicBlock *MultipleDestSentinel = (BasicBlock*)(intptr_t)~0ULL;
+  Constant *OnlyVal = nullptr;
+  Constant *MultipleDestVal = (Constant *)(intptr_t)~0ULL;
 
   unsigned PredWithKnownSuccessor = 0;
   for (const auto &PredValue : PredValues) {
@@ -1253,10 +1255,17 @@
     }
 
     // If we have exactly one destination, remember it for efficiency below.
-    if (PredToDestList.empty())
+    if (PredToDestList.empty()) {
       OnlyDest = DestBB;
-    else if (OnlyDest != DestBB)
-      OnlyDest = MultipleDestSentinel;
+      OnlyVal = Val;
+    } else {
+      if (OnlyDest != DestBB)
+        OnlyDest = MultipleDestSentinel;
+      // It possible we have same destination, but different value, e.g. default
+      // case in switchinst.
+      if (Val != OnlyVal)
+        OnlyVal = MultipleDestVal;
+    }
 
     // We know where this predecessor will go.
     PredWithKnownSuccessor++;
@@ -1297,8 +1306,13 @@
       auto *CondInst = dyn_cast<Instruction>(Cond);
       if (CondInst && CondInst->use_empty())
         CondInst->eraseFromParent();
-      // FIXME: in case this instruction is defined in the current BB and it
-      // resolves to a single value from all predecessors, we can do RAUW.
+      else if (CondInst && OnlyVal && OnlyVal != MultipleDestVal &&
+               CondInst->getParent() == BB) {
+        // If the fact we just learned is the same value for all uses of the
+        // condition, replace it with a constant value
+        CondInst->replaceAllUsesWith(OnlyVal);
+        CondInst->eraseFromParent();
+      }
       return true;
     }
   }


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