[llvm] [InstCombine] Improve `(icmp pred (and X, Y), ...)` fold. (PR #66787)

[Nikita Popov via llvm-commits]

================
@@ -233,56 +233,116 @@ class LLVM_LIBRARY_VISIBILITY InstCombiner {
                                                 PatternMatch::m_Value()));
   }
 
-  /// Return true if the specified value is free to invert (apply ~ to).
-  /// This happens in cases where the ~ can be eliminated.  If WillInvertAllUses
-  /// is true, work under the assumption that the caller intends to remove all
-  /// uses of V and only keep uses of ~V.
-  ///
-  /// See also: canFreelyInvertAllUsersOf()
-  static bool isFreeToInvert(Value *V, bool WillInvertAllUses,
-                             unsigned Depth = 0) {
+  /// Return nonnull value if V is free to invert (with condition) regarding
+  /// WillInvertAllUses.
+  /// If Builder is nonnull, it will return a simplified ~V
+  /// If builder is null, it will return an arbitrary nonnull value (not
+  /// dereferenceable).
+  static Value *getFreeInverted(Value *V, bool WillInvertAllUses,
+                                BuilderTy *Builder, unsigned Depth = 0) {
+    static Value *const kNonNull = reinterpret_cast<Value *>(uintptr_t(1));
     // ~(~(X)) -> X.
-    if (match(V, m_Not(PatternMatch::m_Value())))
-      return true;
+    Value *A, *B;
+    if (match(V, m_Not(PatternMatch::m_Value(A))))
+      return A;
 
+    Constant *C;
     // Constants can be considered to be not'ed values.
-    if (match(V, PatternMatch::m_AnyIntegralConstant()))
-      return true;
+    if (match(V, PatternMatch::m_ImmConstant(C)))
+      return ConstantExpr::getNot(C);
 
     if (Depth++ >= MaxAnalysisRecursionDepth)
-      return false;
+      return nullptr;
 
     // The rest of the cases require that we invert all uses so don't bother
     // doing the analysis if we know we can't use the result.
     if (!WillInvertAllUses)
-      return false;
+      return nullptr;
 
     // Compares can be inverted if all of their uses are being modified to use
     // the ~V.
-    if (isa<CmpInst>(V))
-      return true;
+    if (auto *I = dyn_cast<CmpInst>(V)) {
+      if (Builder != nullptr)
+        return Builder->CreateCmp(I->getInversePredicate(), I->getOperand(0),
+                                  I->getOperand(1));
+      return kNonNull;
+    }
 
-    Value *A, *B;
-    // If `V` is of the form `A + B` then `-1 - V` can be folded into
-    // `~B - A` or `~A - B` if we are willing to invert all of the uses.
-    if (match(V, m_Add(PatternMatch::m_Value(A), PatternMatch::m_Value(B))))
-      return isFreeToInvert(A, A->hasOneUse(), Depth) ||
-             isFreeToInvert(B, B->hasOneUse(), Depth);
+    // If `V` is of the form `A + Constant` then `-1 - V` can be folded into
+    // `(-1 - Constant) - A` if we are willing to invert all of the uses.
+    if (match(V, m_Add(PatternMatch::m_Value(A), PatternMatch::m_Value(B)))) {
+      if (auto *BV = getFreeInverted(B, B->hasOneUse(), Builder, Depth))
+        return Builder ? Builder->CreateSub(BV, A) : kNonNull;
+      if (auto *AV = getFreeInverted(A, A->hasOneUse(), Builder, Depth))
+        return Builder ? Builder->CreateSub(AV, B) : kNonNull;
+      return nullptr;
+    }
 
-    // If `V` is of the form `A - B` then `-1 - V` can be folded into
-    // `~A + B` if we are willing to invert all of the uses.
-    if (match(V, m_Sub(PatternMatch::m_Value(A), PatternMatch::m_Value())))
-      return isFreeToInvert(A, A->hasOneUse(), Depth);
+    // If `V` is of the form `A ^ ~B` then `~(A ^ ~B)` can be folded
+    // into `A ^ B` if we are willing to invert all of the uses.
+    if (match(V, m_Xor(PatternMatch::m_Value(A), PatternMatch::m_Value(B)))) {
+      if (auto *BV = getFreeInverted(B, B->hasOneUse(), Builder, Depth))
+        return Builder ? Builder->CreateXor(A, BV) : kNonNull;
+      if (auto *AV = getFreeInverted(A, A->hasOneUse(), Builder, Depth))
+        return Builder ? Builder->CreateXor(AV, B) : kNonNull;
+      return nullptr;
+    }
 
+    // If `V` is of the form `Constant - A` then `-1 - V` can be folded into
+    // `A + (-1 - Constant)` if we are willing to invert all of the uses.
+    if (match(V, m_Sub(PatternMatch::m_Value(A), PatternMatch::m_Value(B)))) {
+      if (auto *AV = getFreeInverted(A, A->hasOneUse(), Builder, Depth))
+        return Builder ? Builder->CreateAdd(AV, B) : kNonNull;
+      return nullptr;
+    }
+
+    // If `V` is of the form `Constant - A` then `-1 - V` can be folded into
+    // `A + (-1 - Constant)` if we are willing to invert all of the uses.
+    if (match(V, m_AShr(PatternMatch::m_Value(A), PatternMatch::m_Value(B)))) {
+      if (auto *AV = getFreeInverted(A, A->hasOneUse(), Builder, Depth))
+        return Builder ? Builder->CreateAShr(AV, B) : kNonNull;
+      return nullptr;
+    }
+
+    Value *Cond;
+    // LogicOps are special in that we canonicalize them at the cost of an
+    // instruction.
+    bool IsSelect =
+        match(V, m_Select(PatternMatch::m_Value(Cond), PatternMatch::m_Value(A),
+                          PatternMatch::m_Value(B))) &&
+        !match(V, PatternMatch::m_LogicalOp(PatternMatch::m_Value(),
+                                            PatternMatch::m_Value()));
----------------
nikic wrote:

Okay, I see you ended up basically doing the same thing as shouldAvoidAbsorbingNotIntoSelect() in a following commit. Would still be good to use the standard helper for that.

https://github.com/llvm/llvm-project/pull/66787