known_bits.patch

commit 089252cb30d457d731aaddab7a5ee0a4f3fafde7
Author: Nikita Popov <nikita.ppv@gmail.com>
Date:   Fri Mar 6 22:08:14 2020 +0100

    known bits experiment

diff --git llvm/lib/Transforms/InstCombine/InstructionCombining.cpp llvm/lib/Transforms/InstCombine/InstructionCombining.cpp
index eb4541eba13..5cbb88486ac 100644
--- llvm/lib/Transforms/InstCombine/InstructionCombining.cpp
+++ llvm/lib/Transforms/InstCombine/InstructionCombining.cpp
@@ -116,6 +116,8 @@ using namespace llvm::PatternMatch;
 STATISTIC(NumCombined , "Number of insts combined");
 STATISTIC(NumConstProp, "Number of constant folds");
 STATISTIC(NumConstPropKnownBits, "Number of constant folds using known bits");
+STATISTIC(NumConstPropKnownBitsComputed,
+    "Number of times known bits was computed for constant folding");
 STATISTIC(NumDeadInst , "Number of dead inst eliminated");
 STATISTIC(NumSunkInst , "Number of instructions sunk");
 STATISTIC(NumExpand,    "Number of expansions");
@@ -3453,26 +3455,6 @@ bool InstCombiner::run() {
       }
     }
 
-    // In general, it is possible for computeKnownBits to determine all bits in
-    // a value even when the operands are not all constants.
-    Type *Ty = I->getType();
-    if (ExpensiveCombines && !I->use_empty() && Ty->isIntOrIntVectorTy()) {
-      KnownBits Known = computeKnownBits(I, /*Depth*/0, I);
-      if (Known.isConstant()) {
-        Constant *C = ConstantInt::get(Ty, Known.getConstant());
-        LLVM_DEBUG(dbgs() << "IC: ConstFold (all bits known) to: " << *C
-                          << " from: " << *I << '\n');
-
-        // Add operands to the worklist.
-        replaceInstUsesWith(*I, C);
-        ++NumConstPropKnownBits;
-        if (isInstructionTriviallyDead(I, &TLI))
-          eraseInstFromFunction(*I);
-        MadeIRChange = true;
-        continue;
-      }
-    }
-
     // See if we can trivially sink this instruction to a successor basic block.
     if (EnableCodeSinking && I->hasOneUse()) {
       BasicBlock *BB = I->getParent();
@@ -3523,6 +3505,8 @@ bool InstCombiner::run() {
     LLVM_DEBUG(raw_string_ostream SS(OrigI); I->print(SS); OrigI = SS.str(););
     LLVM_DEBUG(dbgs() << "IC: Visiting: " << OrigI << '\n');
 
+    bool OldMadeIRChange = MadeIRChange;
+    MadeIRChange = false;
     if (Instruction *Result = visit(*I)) {
       ++NumCombined;
       // Should we replace the old instruction with a new one?
@@ -3568,6 +3552,31 @@ bool InstCombiner::run() {
         }
       }
       MadeIRChange = true;
+      continue;
+    }
+
+    if (!MadeIRChange) {
+      // In general, it is possible for computeKnownBits to determine all bits
+      // in a value even when the operands are not all constants.
+      Type *Ty = I->getType();
+      if (ExpensiveCombines && !I->use_empty() && Ty->isIntOrIntVectorTy()) {
+        KnownBits Known = computeKnownBits(I, /*Depth*/0, I);
+        ++NumConstPropKnownBitsComputed;
+        if (Known.isConstant()) {
+          Constant *C = ConstantInt::get(Ty, Known.getConstant());
+          LLVM_DEBUG(dbgs() << "IC: ConstFold (all bits known) to: " << *C
+                            << " from: " << *I << '\n');
+
+          // Add operands to the worklist.
+          replaceInstUsesWith(*I, C);
+          ++NumConstPropKnownBits;
+          if (isInstructionTriviallyDead(I, &TLI))
+            eraseInstFromFunction(*I);
+          MadeIRChange = true;
+          continue;
+        }
+      }
+      MadeIRChange = OldMadeIRChange;
     }
   }
 
diff --git llvm/test/Transforms/InstCombine/out-of-bounds-indexes.ll llvm/test/Transforms/InstCombine/out-of-bounds-indexes.ll
index 02be57a4d15..fbab3275cf4 100644
--- llvm/test/Transforms/InstCombine/out-of-bounds-indexes.ll
+++ llvm/test/Transforms/InstCombine/out-of-bounds-indexes.ll
@@ -6,7 +6,7 @@ define i32 @test_out_of_bounds(i32 %a, i1 %x, i1 %y) {
 ; CHECK-LABEL: @test_out_of_bounds(
 ; CHECK-NEXT:  entry:
 ; CHECK-NEXT:    [[AND1:%.*]] = and i32 [[A:%.*]], 3
-; CHECK-NEXT:    tail call void @llvm.assume(i1 false)
+; CHECK-NEXT:    tail call void @llvm.assume(i1 undef)
 ; CHECK-NEXT:    ret i32 [[AND1]]
 ;
 entry:
@@ -20,7 +20,7 @@ entry:
 define i128 @test_non64bit(i128 %a) {
 ; CHECK-LABEL: @test_non64bit(
 ; CHECK-NEXT:    [[AND1:%.*]] = and i128 [[A:%.*]], 3
-; CHECK-NEXT:    tail call void @llvm.assume(i1 false)
+; CHECK-NEXT:    tail call void @llvm.assume(i1 undef)
 ; CHECK-NEXT:    ret i128 [[AND1]]
 ;
   %and1 = and i128 %a, 3
diff --git llvm/test/Transforms/InstCombine/phi-shifts.ll llvm/test/Transforms/InstCombine/phi-shifts.ll
index cc36c9d9e25..186ef1222f9 100644
--- llvm/test/Transforms/InstCombine/phi-shifts.ll
+++ llvm/test/Transforms/InstCombine/phi-shifts.ll
@@ -9,7 +9,7 @@ define i64 @fuzz15217(i1 %cond, i8* %Ptr, i64 %Val) {
 ; CHECK:       two:
 ; CHECK-NEXT:    br label [[END]]
 ; CHECK:       end:
-; CHECK-NEXT:    ret i64 0
+; CHECK-NEXT:    ret i64 undef
 ;
 entry:
   br i1 %cond, label %end, label %two