[PATCH] D99276: [ValueTracking] peek through min/max to find isKnownToBeAPowerOfTwo

[Sanjay Patel via Phabricator via llvm-commits]

spatel created this revision.
spatel added reviewers: lebedev.ri, nikic, RKSimon.
Herald added subscribers: dexonsmith, hiraditya, mcrosier.
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This is similar to the `select` logic just ahead of the new code. Min/max choose exactly one value from the inputs, so if both of those are a power-of-2, then the result must be a power-of-2. This might help with D98152 <https://reviews.llvm.org/D98152>, but we likely still need other pieces of the puzzle to avoid regressions.

The part I'm not sure about is the diff in PatternMatch.h. Without that, I get a pile of errors like:
error: assigning to 'llvm::Value *' from incompatible type 'const llvm::Value *'

...and I haven't been able to find a combination of `const` to avoid it. I did try `const_cast<Value *>(V)` in the caller code, and that also works. Is that better or worse?


https://reviews.llvm.org/D99276

Files:
  llvm/include/llvm/IR/PatternMatch.h
  llvm/lib/Analysis/ValueTracking.cpp
  llvm/test/Transforms/InstSimplify/AndOrXor.ll


Index: llvm/test/Transforms/InstSimplify/AndOrXor.ll
===================================================================
--- llvm/test/Transforms/InstSimplify/AndOrXor.ll
+++ llvm/test/Transforms/InstSimplify/AndOrXor.ll
@@ -97,9 +97,7 @@
 ; CHECK-NEXT:    [[SHX:%.*]] = shl i32 2, [[X:%.*]]
 ; CHECK-NEXT:    [[SHY:%.*]] = shl i32 32, [[Y:%.*]]
 ; CHECK-NEXT:    [[M:%.*]] = call i32 @llvm.smax.i32(i32 [[SHX]], i32 [[SHY]])
-; CHECK-NEXT:    [[NEG:%.*]] = sub i32 0, [[M]]
-; CHECK-NEXT:    [[R:%.*]] = and i32 [[M]], [[NEG]]
-; CHECK-NEXT:    ret i32 [[R]]
+; CHECK-NEXT:    ret i32 [[M]]
 ;
   %shx = shl i32 2, %x
   %shy = shl i32 32, %y
@@ -133,12 +131,7 @@
 
 define <2 x i32> @pow2_decrement_min_vec(<2 x i32> %x, <2 x i32> %y) {
 ; CHECK-LABEL: @pow2_decrement_min_vec(
-; CHECK-NEXT:    [[P1:%.*]] = and <2 x i32> [[X:%.*]], <i32 2048, i32 2048>
-; CHECK-NEXT:    [[P2:%.*]] = shl <2 x i32> <i32 1, i32 1>, [[Y:%.*]]
-; CHECK-NEXT:    [[M:%.*]] = call <2 x i32> @llvm.umin.v2i32(<2 x i32> [[P1]], <2 x i32> [[P2]])
-; CHECK-NEXT:    [[A:%.*]] = add <2 x i32> [[M]], <i32 -1, i32 -1>
-; CHECK-NEXT:    [[R:%.*]] = and <2 x i32> [[M]], [[A]]
-; CHECK-NEXT:    ret <2 x i32> [[R]]
+; CHECK-NEXT:    ret <2 x i32> zeroinitializer
 ;
   %p1 = and <2 x i32> %x, <i32 2048, i32 2048>
   %p2 = shl <2 x i32> <i32 1, i32 1>, %y
Index: llvm/lib/Analysis/ValueTracking.cpp
===================================================================
--- llvm/lib/Analysis/ValueTracking.cpp
+++ llvm/lib/Analysis/ValueTracking.cpp
@@ -1977,6 +1977,12 @@
     return isKnownToBeAPowerOfTwo(SI->getTrueValue(), OrZero, Depth, Q) &&
            isKnownToBeAPowerOfTwo(SI->getFalseValue(), OrZero, Depth, Q);
 
+  // Peek through min/max.
+  if (match(V, m_MaxOrMin(m_Value(X), m_Value(Y)))) {
+    return isKnownToBeAPowerOfTwo(X, OrZero, Depth, Q) &&
+           isKnownToBeAPowerOfTwo(Y, OrZero, Depth, Q);
+  }
+
   if (OrZero && match(V, m_And(m_Value(X), m_Value(Y)))) {
     // A power of two and'd with anything is a power of two or zero.
     if (isKnownToBeAPowerOfTwo(X, /*OrZero*/ true, Depth, Q) ||
Index: llvm/include/llvm/IR/PatternMatch.h
===================================================================
--- llvm/include/llvm/IR/PatternMatch.h
+++ llvm/include/llvm/IR/PatternMatch.h
@@ -1735,10 +1735,10 @@
       return false;
     // At this point we have a select conditioned on a comparison.  Check that
     // it is the values returned by the select that are being compared.
-    Value *TrueVal = SI->getTrueValue();
-    Value *FalseVal = SI->getFalseValue();
-    Value *LHS = Cmp->getOperand(0);
-    Value *RHS = Cmp->getOperand(1);
+    auto *TrueVal = SI->getTrueValue();
+    auto *FalseVal = SI->getFalseValue();
+    auto *LHS = Cmp->getOperand(0);
+    auto *RHS = Cmp->getOperand(1);
     if ((TrueVal != LHS || FalseVal != RHS) &&
         (TrueVal != RHS || FalseVal != LHS))
       return false;


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