[InstCombine] complete (X << Z) / (Y << Z) --> X / Y

[Chenbing.Zheng]

Summary

Add one more situation for this fold
For unsigned div, 'nsw' on both shifts + 'nuw' on the dividend.

Alive2: https://alive2.llvm.org/ce/z/sELF76

[spatel]

LGTM - thanks!

llvm/lib/Transforms/InstCombine/InstCombineMulDivRem.cpp

static Instruction *foldIDivShl(BinaryOperator &I,
 
   // With appropriate no-wrap constraints, remove a common factor in the
   // dividend and divisor that is disguised as a left-shift amount.
   if (match(Op0, m_Shl(m_Value(X), m_Value(Z))) &&
       match(Op1, m_Shl(m_Value(Y), m_Specific(Z)))) {
     auto *Shl0 = cast<OverflowingBinaryOperator>(Op0);
     auto *Shl1 = cast<OverflowingBinaryOperator>(Op1);
 
-    // For unsigned div, we need 'nuw' on both shifts.
+    // For unsigned div, we need 'nuw' on both shifts or
+    // 'nsw' on both shifts + 'nuw' on the dividend.
     // (X << Z) / (Y << Z) --> X / Y
-    if (!IsSigned && Shl0->hasNoUnsignedWrap() && Shl1->hasNoUnsignedWrap())
+    if (!IsSigned &&
+        ((Shl0->hasNoUnsignedWrap() && Shl1->hasNoUnsignedWrap()) ||
+         (Shl0->hasNoUnsignedWrap() && Shl0->hasNoSignedWrap() &&
+          Shl1->hasNoSignedWrap())))
       Ret = BinaryOperator::CreateUDiv(X, Y);
 
     // For signed div, we need 'nsw' on both shifts + 'nuw' on the divisor.
     // (X << Z) / (Y << Z) --> X / Y
     if (IsSigned && Shl0->hasNoSignedWrap() && Shl1->hasNoSignedWrap() &&
         Shl1->hasNoUnsignedWrap())
       Ret = BinaryOperator::CreateSDiv(X, Y);
   }

llvm/test/Transforms/InstCombine/div-shift.ll

 ; CHECK-NEXT:    ret i8 [[D]]
 ;
   %xz = shl nsw i8 %x, %z
   %yz = shl nuw i8 %y, %z
   %d = udiv i8 %xz, %yz
   ret i8 %d
 }
 
-; TODO: This could fold.
-
 define i8 @udiv_shl_shl_nuw_nsw2(i8 %x, i8 %y, i8 %z) {
 ; CHECK-LABEL: @udiv_shl_shl_nuw_nsw2(
-; CHECK-NEXT:    [[XZ:%.*]] = shl nuw nsw i8 [[X:%.*]], [[Z:%.*]]
-; CHECK-NEXT:    [[YZ:%.*]] = shl nsw i8 [[Y:%.*]], [[Z]]
-; CHECK-NEXT:    [[D:%.*]] = udiv i8 [[XZ]], [[YZ]]
+; CHECK-NEXT:    [[D:%.*]] = udiv i8 [[X:%.*]], [[Y:%.*]]
 ; CHECK-NEXT:    ret i8 [[D]]
 ;
   %xz = shl nuw nsw i8 %x, %z
   %yz = shl nsw i8 %y, %z
   %d = udiv i8 %xz, %yz
   ret i8 %d
 }