Use LVI to eliminate conditional branches where we've tested a relate…

…d condition previously. Update tests for this change. This fixes PR5652. git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@112270 91177308-0d34-0410-b5e6-96231b3b80d8
zoren · Aug 27, 2010 · 660cab3 · 660cab3
1 parent 67ef74e
commit 660cab3
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Showing 4 changed files with 53 additions and 6 deletions.
diff --git a/lib/Analysis/LazyValueInfo.cpp b/lib/Analysis/LazyValueInfo.cpp
@@ -656,7 +656,8 @@ LVILatticeVal LVIQuery::getEdgeValue(BasicBlock *BBFrom, BasicBlock *BBTo) {
 
           // Figure out the possible values of the query BEFORE this branch.  
           LVILatticeVal InBlock = getBlockValue(BBFrom);
-          if (!InBlock.isConstantRange()) return InBlock;
+          if (!InBlock.isConstantRange())
+            return LVILatticeVal::getRange(TrueValues);
 
           // Find all potential values that satisfy both the input and output
           // conditions.

diff --git a/lib/Transforms/Scalar/JumpThreading.cpp b/lib/Transforms/Scalar/JumpThreading.cpp
@@ -669,6 +669,45 @@ bool JumpThreading::ProcessBlock(BasicBlock *BB) {
           }
       }
     }
+
+    // For a comparison where the LHS is outside this block, it's possible
+    // that we've branch on it before.  Used LVI to see if we can simplify
+    // the branch based on that.
+    BranchInst *CondBr = dyn_cast<BranchInst>(BB->getTerminator());
+    Constant *CondConst = dyn_cast<Constant>(CondCmp->getOperand(1));
+    if (LVI && CondBr && CondConst && CondBr->isConditional() &&
+        (!isa<Instruction>(CondCmp->getOperand(0)) ||
+         cast<Instruction>(CondCmp->getOperand(0))->getParent() != BB)) {
+      // For predecessor edge, determine if the comparison is true or false
+      // on that edge.  If they're all true or all false, we can simplify the
+      // branch.
+      // FIXME: We could handle mixed true/false by duplicating code.
+      unsigned Trues = 0, Falses = 0, predcount = 0;
+      for (pred_iterator PI = pred_begin(BB), PE = pred_end(BB);PI != PE; ++PI){
+        ++predcount;
+        LazyValueInfo::Tristate Ret =
+          LVI->getPredicateOnEdge(CondCmp->getPredicate(), 
+                                  CondCmp->getOperand(0), CondConst, *PI, BB);
+        if (Ret == LazyValueInfo::True)
+          ++Trues;
+        else if (Ret == LazyValueInfo::False)
+          ++Falses;
+      }
+
+      // If we can determine the branch direction statically, converted
+      // the conditional branch to an unconditional one.
+      if (Trues && Trues == predcount) {
+        RemovePredecessorAndSimplify(CondBr->getSuccessor(1), BB, TD);
+        BranchInst::Create(CondBr->getSuccessor(0), CondBr);
+        CondBr->eraseFromParent();
+        return true;
+      } else if (Falses && Falses == predcount) {
+        RemovePredecessorAndSimplify(CondBr->getSuccessor(0), BB, TD);
+        BranchInst::Create(CondBr->getSuccessor(1), CondBr);
+        CondBr->eraseFromParent();
+        return true;
+      }
+    }
   }
 
   // Check for some cases that are worth simplifying.  Right now we want to look

diff --git a/test/Transforms/JumpThreading/basic.ll b/test/Transforms/JumpThreading/basic.ll
@@ -147,11 +147,17 @@ define i32 @test6(i32 %A) {
 ; CHECK: @test6
 	%tmp455 = icmp eq i32 %A, 42
 	br i1 %tmp455, label %BB1, label %BB2
-
-BB2:
+
+; CHECK: call i32 @f2()
+; CHECK-NEXT: ret i32 3
+
 ; CHECK: call i32 @f1()
-; CHECK-NEXT: call void @f3()
-; CHECK-NEXT: ret i32 4
+; CHECK-NOT: br
+; CHECK: call void @f3()
+; CHECK-NOT: br
+; CHECK: ret i32 4
+
+BB2:
 	call i32 @f1()
 	br label %BB1
 

diff --git a/test/Transforms/JumpThreading/lvi-load.ll b/test/Transforms/JumpThreading/lvi-load.ll
@@ -1,4 +1,4 @@
-; RUN: opt -S -jump-threading -enable-jump-threading-lvi < %s | FileCheck %s
+; RUN: opt -S -jump-threading -enable-jump-threading-lvi -dce < %s | FileCheck %s
 target datalayout = "e-p:64:64:64-i1:8:8-i8:8:8-i16:16:16-i32:32:32-i64:64:64-f32:32:32-f64:64:64-v64:64:64-v128:128:128-a0:0:64-s0:64:64-f80:128:128-n8:16:32:64"
 target triple = "x86_64-apple-darwin10.4"
 
@@ -25,6 +25,7 @@ bb.i:                                             ; preds = %entry
   %toBoolnot.i.i = icmp ult i8 %1, 21             ; <i1> [#uses=1]
   br i1 %toBoolnot.i.i, label %bb6.i.i, label %_ZN4llvm8dyn_castINS_11InstructionEPNS_5ValueEEENS_10cast_rettyIT_T0_E8ret_typeERKS6_.exit
 
+; CHECK-NOT: assert
 bb6.i.i:                                          ; preds = %bb.i
   tail call void @__assert_rtn(i8* getelementptr inbounds ([5 x i8]* @_ZZN4llvm4castINS_11InstructionEPNS_5ValueEEENS_10cast_rettyIT_T0_E8ret_typeERKS6_E8__func__, i64 0, i64 0), i8* getelementptr inbounds ([31 x i8]* @.str, i64 0, i64 0), i32 202, i8* getelementptr inbounds ([59 x i8]* @.str1, i64 0, i64 0)) noreturn
   unreachable