Merging r322006:

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r322006 | davide | 2018-01-08 17:34:06 +0100 (Mon, 08 Jan 2018) | 19 lines

[CVP] Replace incoming values from unreachable blocks with undef.

This is an attempt of fixing PR35807.
Due to the non-standard definition of dominance in LLVM, where uses in
unreachable blocks are dominated by anything, you can have, in an
unreachable block:

  %patatino = OP1 %patatino, CONSTANT

When `SimplifyInstruction` receives a PHI where an incoming value is of
the aforementioned form, in some cases, loops indefinitely.

What I propose here instead is keeping track of the incoming values
from unreachable blocks, and replacing them with undef. It fixes this
case, and it seems to be good regardless (even if we can't prove that
the value is constant, as it's coming from an unreachable block, we
can ignore it).

Differential Revision:  https://reviews.llvm.org/D41812
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git-svn-id: https://llvm.org/svn/llvm-project/llvm/branches/release_60@323738 91177308-0d34-0410-b5e6-96231b3b80d8

diff --git a/lib/Transforms/Scalar/CorrelatedValuePropagation.cpp b/lib/Transforms/Scalar/CorrelatedValuePropagation.cpp
index 8f468ebf894911921035bc867b446cb7729c7485..7b898a3943d5e8eaff8c76fe644a7dbc1a3a1c14 100644 (file)
--- a/lib/Transforms/Scalar/CorrelatedValuePropagation.cpp
+++ b/lib/Transforms/Scalar/CorrelatedValuePropagation.cpp
@@ -14,6 +14,7 @@
 #include "llvm/Transforms/Scalar/CorrelatedValuePropagation.h"
 #include "llvm/ADT/DepthFirstIterator.h"
 #include "llvm/ADT/Optional.h"
+#include "llvm/ADT/PostOrderIterator.h"
 #include "llvm/ADT/SmallVector.h"
 #include "llvm/ADT/Statistic.h"
 #include "llvm/Analysis/GlobalsModRef.h"
@@ -120,8 +121,8 @@ static bool processSelect(SelectInst *S, LazyValueInfo *LVI) {
   return true;
 }
 
-static bool processPHI(PHINode *P, LazyValueInfo *LVI,
-                       const SimplifyQuery &SQ) {
+static bool processPHI(PHINode *P, LazyValueInfo *LVI, const SimplifyQuery &SQ,
+                       DenseSet<BasicBlock *> &ReachableBlocks) {
   bool Changed = false;
 
   BasicBlock *BB = P->getParent();
@@ -129,7 +130,18 @@ static bool processPHI(PHINode *P, LazyValueInfo *LVI,
     Value *Incoming = P->getIncomingValue(i);
     if (isa<Constant>(Incoming)) continue;
 
-    Value *V = LVI->getConstantOnEdge(Incoming, P->getIncomingBlock(i), BB, P);
+    // If the incoming value is coming from an unreachable block, replace
+    // it with undef and go on. This is good for two reasons:
+    // 1) We skip an LVI query for an unreachable block
+    // 2) We transform the incoming value so that the code below doesn't
+    //    mess around with IR in unreachable blocks.
+    BasicBlock *IncomingBB = P->getIncomingBlock(i);
+    if (!ReachableBlocks.count(IncomingBB)) {
+      P->setIncomingValue(i, UndefValue::get(P->getType()));
+      continue;
+    }
+
+    Value *V = LVI->getConstantOnEdge(Incoming, IncomingBB, BB, P);
 
     // Look if the incoming value is a select with a scalar condition for which
     // LVI can tells us the value. In that case replace the incoming value with
@@ -561,6 +573,14 @@ static Constant *getConstantAt(Value *V, Instruction *At, LazyValueInfo *LVI) {
 
 static bool runImpl(Function &F, LazyValueInfo *LVI, const SimplifyQuery &SQ) {
   bool FnChanged = false;
+
+  // Compute reachability from the entry block of this function via an RPO
+  // walk. We use this information when processing PHIs.
+  DenseSet<BasicBlock *> ReachableBlocks;
+  ReversePostOrderTraversal<Function *> RPOT(&F);
+  for (BasicBlock *BB : RPOT)
+    ReachableBlocks.insert(BB);
+
   // Visiting in a pre-order depth-first traversal causes us to simplify early
   // blocks before querying later blocks (which require us to analyze early
   // blocks).  Eagerly simplifying shallow blocks means there is strictly less
@@ -575,7 +595,7 @@ static bool runImpl(Function &F, LazyValueInfo *LVI, const SimplifyQuery &SQ) {
         BBChanged |= processSelect(cast<SelectInst>(II), LVI);
         break;
       case Instruction::PHI:
-        BBChanged |= processPHI(cast<PHINode>(II), LVI, SQ);
+        BBChanged |= processPHI(cast<PHINode>(II), LVI, SQ, ReachableBlocks);
         break;
       case Instruction::ICmp:
       case Instruction::FCmp:

diff --git a/test/Transforms/CorrelatedValuePropagation/pr35807.ll b/test/Transforms/CorrelatedValuePropagation/pr35807.ll
new file mode 100644 (file)
index 0000000..8f72b59
--- /dev/null
+++ b/test/Transforms/CorrelatedValuePropagation/pr35807.ll
@@ -0,0 +1,65 @@
+; NOTE: Assertions have been autogenerated by utils/update_test_checks.py
+; RUN: opt -correlated-propagation -S %s | FileCheck %s
+
+target triple = "x86_64-apple-darwin17.4.0"
+
+define void @patatino() {
+; CHECK-LABEL: @patatino(
+; CHECK-NEXT:    br i1 undef, label [[BB3:%.*]], label [[BB4:%.*]]
+; CHECK:       bb3:
+; CHECK-NEXT:    br label [[BB3]]
+; CHECK:       bb4:
+; CHECK-NEXT:    br i1 undef, label [[BB40:%.*]], label [[BB22:%.*]]
+; CHECK:       bb7:
+; CHECK-NEXT:    br label [[BB14:%.*]]
+; CHECK:       bb12:
+; CHECK-NEXT:    br label [[BB14]]
+; CHECK:       bb14:
+; CHECK-NEXT:    [[TMP19:%.*]] = icmp sgt i32 undef, undef
+; CHECK-NEXT:    [[TMP20:%.*]] = select i1 [[TMP19]], i64 [[TMP20]], i64 0
+; CHECK-NEXT:    br i1 undef, label [[BB40]], label [[BB7:%.*]]
+; CHECK:       bb22:
+; CHECK-NEXT:    br label [[BB24:%.*]]
+; CHECK:       bb24:
+; CHECK-NEXT:    br label [[BB32:%.*]]
+; CHECK:       bb32:
+; CHECK-NEXT:    br i1 undef, label [[BB40]], label [[BB24]]
+; CHECK:       bb40:
+; CHECK-NEXT:    ret void
+;
+  br i1 undef, label %bb3, label %bb4
+
+bb3:
+  br label %bb3
+
+bb4:
+  br i1 undef, label %bb40, label %bb22
+
+bb7:
+  br label %bb14
+
+bb12:
+  br label %bb14
+
+; This block is unreachable. Due to the non-standard definition of
+; dominance in LLVM where uses in unreachable blocks are dominated
+; by anything, it contains an instruction of the form
+; %def = OP %def, %something
+bb14:
+  %tmp19 = icmp sgt i32 undef, undef
+  %tmp20 = select i1 %tmp19, i64 %tmp20, i64 0
+  br i1 undef, label %bb40, label %bb7
+
+bb22:
+  br label %bb24
+
+bb24:
+  br label %bb32
+
+bb32:
+  br i1 undef, label %bb40, label %bb24
+
+bb40:
+  %tmp41 = phi i64 [ 4, %bb4 ], [ %tmp20, %bb14 ], [ undef, %bb32 ]
+  ret void
+}