[llvm] [AggressiveInstCombine] Match long high-half multiply (PR #168396)

[Cullen Rhodes via llvm-commits]

================
@@ -1466,6 +1466,307 @@ static bool foldLibCalls(Instruction &I, TargetTransformInfo &TTI,
   return false;
 }
 
+/// Match high part of long multiplication.
+///
+/// Considering a multiply made up of high and low parts, we can split the
+/// multiply into:
+///  x * y == (xh*T + xl) * (yh*T + yl)
+/// where xh == x>>32 and xl == x & 0xffffffff. T = 2^32.
+/// This expands to
+///  xh*yh*T*T + xh*yl*T + xl*yh*T + xl*yl
+/// which can be drawn as
+/// [  xh*yh  ]
+///      [  xh*yl  ]
+///      [  xl*yh  ]
+///           [  xl*yl  ]
+/// We are looking for the "high" half, which is xh*yh + xh*yl>>32 + xl*yh>>32 +
+/// some carrys. The carry makes this difficult and there are multiple ways of
+/// representing it. The ones we attempt to support here are:
+///  Carry:  xh*yh + carry + lowsum
+///          carry = lowsum < xh*yl ? 0x1000000 : 0
+///          lowsum = xh*yl + xl*yh + (xl*yl>>32)
+///  Ladder: xh*yh + c2>>32 + c3>>32
+///          c2 = xh*yl + (xl*yl>>32); c3 = c2&0xffffffff + xl*yh
+///  Carry4: xh*yh + carry + crosssum>>32 + (xl*yl + crosssum&0xffffffff) >> 32
+///          crosssum = xh*yl + xl*yh
+///          carry = crosssum < xh*yl ? 0x1000000 : 0
+///  Ladder4: xh*yh + (xl*yh)>>32 + (xh*yl)>>32 + low>>32;
+///          low = (xl*yl)>>32 + (xl*yh)&0xffffffff + (xh*yl)&0xffffffff
+///
+/// They all start by matching xh*yh + 2 or 3 other operands. The bottom of the
+/// tree is xh*yh, xh*yl, xl*yh and xl*yl.
+static bool foldMulHigh(Instruction &I) {
+  Type *Ty = I.getType();
+  if (!Ty->isIntOrIntVectorTy())
+    return false;
+
+  unsigned BW = Ty->getScalarSizeInBits();
----------------
c-rhodes wrote:

nit: please spell it out
```suggestion
  unsigned BitWidth = Ty->getScalarSizeInBits();
```

https://github.com/llvm/llvm-project/pull/168396