[llvm] 05347f8 - [ValueTracking] Compute knownbits from `(icmp upred (add/sub nuw X, Y), C)`

[Noah Goldstein via llvm-commits]

Author: Noah Goldstein
Date: 2024-05-16T13:03:32-05:00
New Revision: 05347f8c2fde0cde2fd12f1ec3611eeaf666ece0

URL: https://github.com/llvm/llvm-project/commit/05347f8c2fde0cde2fd12f1ec3611eeaf666ece0
DIFF: https://github.com/llvm/llvm-project/commit/05347f8c2fde0cde2fd12f1ec3611eeaf666ece0.diff

LOG: [ValueTracking] Compute knownbits from `(icmp upred (add/sub nuw X, Y), C)`

`(icmp ule/ult (add nuw X, Y), C)` implies both `(icmp ule/ult X, C)` and
`(icmp ule/ult Y, C)`. We can use this to deduce leading zeros in `X`/`Y`.

`(icmp uge/ugt (sub nuw X, Y), C)` implies `(icmp uge/uge X, C)` . We
can use this to deduce leading ones in `X`.

Proofs: https://alive2.llvm.org/ce/z/sc5k22

Closes #87180

Added: 
    

Modified: 
    llvm/lib/Analysis/ValueTracking.cpp
    llvm/test/Transforms/InstCombine/known-bits.ll

Removed: 
    


################################################################################
diff  --git a/llvm/lib/Analysis/ValueTracking.cpp b/llvm/lib/Analysis/ValueTracking.cpp
index e07d14ccbb85c..e8c5f9b3dc25d 100644
--- a/llvm/lib/Analysis/ValueTracking.cpp
+++ b/llvm/lib/Analysis/ValueTracking.cpp
@@ -706,17 +706,22 @@ static void computeKnownBitsFromCmp(const Value *V, CmpInst::Predicate Pred,
           LHSRange = LHSRange.sub(*Offset);
         Known = Known.unionWith(LHSRange.toKnownBits());
       }
-      // X & Y u> C -> X u> C && Y u> C
-      if ((Pred == ICmpInst::ICMP_UGT || Pred == ICmpInst::ICMP_UGE) &&
-          match(LHS, m_c_And(m_V, m_Value()))) {
-        Known.One.setHighBits(
-            (*C + (Pred == ICmpInst::ICMP_UGT)).countLeadingOnes());
+      if (Pred == ICmpInst::ICMP_UGT || Pred == ICmpInst::ICMP_UGE) {
+        // X & Y u> C     -> X u> C && Y u> C
+        // X nuw- Y u> C  -> X u> C
+        if (match(LHS, m_c_And(m_V, m_Value())) ||
+            match(LHS, m_NUWSub(m_V, m_Value())))
+          Known.One.setHighBits(
+              (*C + (Pred == ICmpInst::ICMP_UGT)).countLeadingOnes());
       }
-      // X | Y u< C -> X u< C && Y u< C
-      if ((Pred == ICmpInst::ICMP_ULT || Pred == ICmpInst::ICMP_ULE) &&
-          match(LHS, m_c_Or(m_V, m_Value()))) {
-        Known.Zero.setHighBits(
-            (*C - (Pred == ICmpInst::ICMP_ULT)).countLeadingZeros());
+      if (Pred == ICmpInst::ICMP_ULT || Pred == ICmpInst::ICMP_ULE) {
+        // X | Y u< C    -> X u< C && Y u< C
+        // X nuw+ Y u< C -> X u< C && Y u< C
+        if (match(LHS, m_c_Or(m_V, m_Value())) ||
+            match(LHS, m_c_NUWAdd(m_V, m_Value()))) {
+          Known.Zero.setHighBits(
+              (*C - (Pred == ICmpInst::ICMP_ULT)).countLeadingZeros());
+        }
       }
     }
     break;
@@ -9576,14 +9581,20 @@ void llvm::findValuesAffectedByCondition(
           if (match(A, m_AddLike(m_Value(X), m_ConstantInt())))
             AddAffected(X);
 
-          Value *Y;
-          // X & Y u> C -> X >u C && Y >u C
-          // X | Y u< C -> X u< C && Y u< C
-          if (ICmpInst::isUnsigned(Pred) &&
-              (match(A, m_And(m_Value(X), m_Value(Y))) ||
-               match(A, m_Or(m_Value(X), m_Value(Y))))) {
-            AddAffected(X);
-            AddAffected(Y);
+          if (ICmpInst::isUnsigned(Pred)) {
+            Value *Y;
+            // X & Y u> C    -> X >u C && Y >u C
+            // X | Y u< C    -> X u< C && Y u< C
+            // X nuw+ Y u< C -> X u< C && Y u< C
+            if (match(A, m_And(m_Value(X), m_Value(Y))) ||
+                match(A, m_Or(m_Value(X), m_Value(Y))) ||
+                match(A, m_NUWAdd(m_Value(X), m_Value(Y)))) {
+              AddAffected(X);
+              AddAffected(Y);
+            }
+            // X nuw- Y u> C -> X u> C
+            if (match(A, m_NUWSub(m_Value(X), m_Value())))
+              AddAffected(X);
           }
         }
 

diff  --git a/llvm/test/Transforms/InstCombine/known-bits.ll b/llvm/test/Transforms/InstCombine/known-bits.ll
index c00bdb7870953..82cd24027e4e1 100644
--- a/llvm/test/Transforms/InstCombine/known-bits.ll
+++ b/llvm/test/Transforms/InstCombine/known-bits.ll
@@ -1705,8 +1705,7 @@ define i8 @test_icmp_add(i8 %n, i8 %n2, i8 %other) {
 ; CHECK-NEXT:    [[CMP:%.*]] = icmp ult i8 [[N_ADD]], 32
 ; CHECK-NEXT:    br i1 [[CMP]], label [[IF_THEN:%.*]], label [[IF_ELSE:%.*]]
 ; CHECK:       if.then:
-; CHECK-NEXT:    [[R:%.*]] = and i8 [[N]], 32
-; CHECK-NEXT:    ret i8 [[R]]
+; CHECK-NEXT:    ret i8 0
 ; CHECK:       if.else:
 ; CHECK-NEXT:    ret i8 [[OTHER:%.*]]
 ;
@@ -1757,8 +1756,7 @@ define i8 @test_icmp_add2(i8 %n, i8 %n2, i8 %other) {
 ; CHECK:       if.then:
 ; CHECK-NEXT:    ret i8 [[OTHER:%.*]]
 ; CHECK:       if.else:
-; CHECK-NEXT:    [[R:%.*]] = and i8 [[N]], 32
-; CHECK-NEXT:    ret i8 [[R]]
+; CHECK-NEXT:    ret i8 0
 ;
 entry:
   %n_add = add nsw nuw i8 %n, %n2
@@ -1830,8 +1828,7 @@ define i8 @test_icmp_sub(i8 %n, i8 %n2, i8 %other) {
 ; CHECK-NEXT:    [[CMP:%.*]] = icmp ugt i8 [[N_SUB]], -33
 ; CHECK-NEXT:    br i1 [[CMP]], label [[IF_THEN:%.*]], label [[IF_ELSE:%.*]]
 ; CHECK:       if.then:
-; CHECK-NEXT:    [[R:%.*]] = and i8 [[N]], 32
-; CHECK-NEXT:    ret i8 [[R]]
+; CHECK-NEXT:    ret i8 32
 ; CHECK:       if.else:
 ; CHECK-NEXT:    ret i8 [[OTHER:%.*]]
 ;
@@ -1882,8 +1879,7 @@ define i8 @test_icmp_sub2(i8 %n, i8 %n2, i8 %other) {
 ; CHECK:       if.then:
 ; CHECK-NEXT:    ret i8 [[OTHER:%.*]]
 ; CHECK:       if.else:
-; CHECK-NEXT:    [[R:%.*]] = and i8 [[N]], 32
-; CHECK-NEXT:    ret i8 [[R]]
+; CHECK-NEXT:    ret i8 32
 ;
 entry:
   %n_sub = sub nuw i8 %n, %n2