[InstCombine] Handle equality comparison when flooring by constant 2

Support `icmp eq` when reducing signed divisions by power of 2 to
arithmetic shift right, as `icmp ugt` may have been canonicalized
into `icmp eq` by the time additions are folded into `ashr`.

Fixes: #73622.

Proof: https://alive2.llvm.org/ce/z/8-eUdb.

llvm/lib/Transforms/InstCombine/InstCombineAddSub.cpp

@@ -1234,18 +1234,28 @@ static Instruction *foldAddToAshr(BinaryOperator &Add) {
     return nullptr;
 
   // Rounding is done by adding -1 if the dividend (X) is negative and has any
-  // low bits set. The canonical pattern for that is an "ugt" compare with SMIN:
-  // sext (icmp ugt (X & (DivC - 1)), SMIN)
-  const APInt *MaskC;
+  // low bits set. It recognizes two canonical patterns:
+  // 1. For an 'ugt' cmp with the signed minimum value (SMIN), the
+  //    pattern is: sext (icmp ugt (X & (DivC - 1)), SMIN).
+  // 2. For an 'eq' cmp, the pattern's: sext (icmp eq X & (SMIN + 1), SMIN + 1).
+  // Note that, by the time we end up here, if possible, ugt has been
+  // canonicalized into eq.
+  const APInt *MaskC, *MaskCCmp;
   ICmpInst::Predicate Pred;
   if (!match(Add.getOperand(1),
              m_SExt(m_ICmp(Pred, m_And(m_Specific(X), m_APInt(MaskC)),
-                           m_SignMask()))) ||
-      Pred != ICmpInst::ICMP_UGT)
+                           m_APInt(MaskCCmp)))))
+    return nullptr;
+
+  if ((Pred != ICmpInst::ICMP_UGT || !MaskCCmp->isSignMask()) &&
+      (Pred != ICmpInst::ICMP_EQ || *MaskCCmp != *MaskC))
     return nullptr;
 
   APInt SMin = APInt::getSignedMinValue(Add.getType()->getScalarSizeInBits());
-  if (*MaskC != (SMin | (*DivC - 1)))
+  bool IsMaskValid = Pred == ICmpInst::ICMP_UGT
+                         ? (*MaskC == (SMin | (*DivC - 1)))
+                         : (*DivC == 2 && *MaskC == SMin + 1);
+  if (!IsMaskValid)
     return nullptr;
 
   // (X / DivC) + sext ((X & (SMin | (DivC - 1)) >u SMin) --> X >>s log2(DivC)

llvm/test/Transforms/InstCombine/add.ll

@@ -2700,6 +2700,70 @@ define i32 @floor_sdiv(i32 %x) {
   ret i32 %r
 }
 
+define i8 @floor_sdiv_by_2(i8 %x) {
+; CHECK-LABEL: @floor_sdiv_by_2(
+; CHECK-NEXT:    [[RV:%.*]] = ashr i8 [[X:%.*]], 1
+; CHECK-NEXT:    ret i8 [[RV]]
+;
+  %div = sdiv i8 %x, 2
+  %and = and i8 %x, -127
+  %icmp = icmp eq i8 %and, -127
+  %sext = sext i1 %icmp to i8
+  %rv = add nsw i8 %div, %sext
+  ret i8 %rv
+}
+
+define i8 @floor_sdiv_by_2_wrong_mask(i8 %x) {
+; CHECK-LABEL: @floor_sdiv_by_2_wrong_mask(
+; CHECK-NEXT:    [[DIV:%.*]] = sdiv i8 [[X:%.*]], 2
+; CHECK-NEXT:    [[AND:%.*]] = and i8 [[X]], 127
+; CHECK-NEXT:    [[ICMP:%.*]] = icmp eq i8 [[AND]], 127
+; CHECK-NEXT:    [[SEXT:%.*]] = sext i1 [[ICMP]] to i8
+; CHECK-NEXT:    [[RV:%.*]] = add nsw i8 [[DIV]], [[SEXT]]
+; CHECK-NEXT:    ret i8 [[RV]]
+;
+  %div = sdiv i8 %x, 2
+  %and = and i8 %x, 127
+  %icmp = icmp eq i8 %and, 127
+  %sext = sext i1 %icmp to i8
+  %rv = add nsw i8 %div, %sext
+  ret i8 %rv
+}
+
+define i8 @floor_sdiv_by_2_wrong_constant(i8 %x) {
+; CHECK-LABEL: @floor_sdiv_by_2_wrong_constant(
+; CHECK-NEXT:    [[DIV:%.*]] = sdiv i8 [[X:%.*]], 4
+; CHECK-NEXT:    [[AND:%.*]] = and i8 [[X]], -125
+; CHECK-NEXT:    [[ICMP:%.*]] = icmp eq i8 [[AND]], -125
+; CHECK-NEXT:    [[SEXT:%.*]] = sext i1 [[ICMP]] to i8
+; CHECK-NEXT:    [[RV:%.*]] = add nsw i8 [[DIV]], [[SEXT]]
+; CHECK-NEXT:    ret i8 [[RV]]
+;
+  %div = sdiv i8 %x, 4
+  %and = and i8 %x, -125
+  %icmp = icmp eq i8 %and, -125
+  %sext = sext i1 %icmp to i8
+  %rv = add nsw i8 %div, %sext
+  ret i8 %rv
+}
+
+define i8 @floor_sdiv_by_2_wrong_cast(i8 %x) {
+; CHECK-LABEL: @floor_sdiv_by_2_wrong_cast(
+; CHECK-NEXT:    [[DIV:%.*]] = sdiv i8 [[X:%.*]], 2
+; CHECK-NEXT:    [[AND:%.*]] = and i8 [[X]], -127
+; CHECK-NEXT:    [[ICMP:%.*]] = icmp eq i8 [[AND]], -127
+; CHECK-NEXT:    [[SEXT:%.*]] = zext i1 [[ICMP]] to i8
+; CHECK-NEXT:    [[RV:%.*]] = add nsw i8 [[DIV]], [[SEXT]]
+; CHECK-NEXT:    ret i8 [[RV]]
+;
+  %div = sdiv i8 %x, 2
+  %and = and i8 %x, -127
+  %icmp = icmp eq i8 %and, -127
+  %sext = zext i1 %icmp to i8
+  %rv = add nsw i8 %div, %sext
+  ret i8 %rv
+}
+
 ; vectors work too and commute is handled by complexity-based canonicalization
 
 define <2 x i32> @floor_sdiv_vec_commute(<2 x i32> %x) {