[llvm] [RISCV] Introduce new AND combine to expose additional load narrowing opportunities (PR #170483)

[Alex Bradbury via llvm-commits]

================
@@ -16607,6 +16607,38 @@ static SDValue performANDCombine(SDNode *N,
   SelectionDAG &DAG = DCI.DAG;
 
   SDValue N0 = N->getOperand(0);
+  SDValue N1 = N->getOperand(1);
+
+  // Sometimes a mask is applied after a shift. If that shift was fed by a
+  // load, there is sometimes the opportunity to narrow the load, which is
+  // hidden by the intermediate shift. Detect that case and commute the
+  // shift/and in order to enable load narrowing.
+  if (N0.getOpcode() == ISD::SHL && N0.hasOneUse() && isa<ConstantSDNode>(N1) &&
+      isa<ConstantSDNode>(N0.getOperand(1))) {
+
+    EVT VT = N->getValueType(0);
+    auto *MaskC = cast<ConstantSDNode>(N1);
+    auto *ShiftC = cast<ConstantSDNode>(N0.getOperand(1));
+
+    uint64_t ShiftAmt = ShiftC->getZExtValue();
+    APInt MaskVal = MaskC->getAPIntValue();
+    // Calculate the mask if it were applied before the shift.
+    APInt InnerMask = MaskVal.lshr(ShiftAmt);
+
+    bool IsNarrowable =
+        InnerMask == 0xff || InnerMask == 0xffff || InnerMask == 0xffffffff;
+
+    if (IsNarrowable && isa<LoadSDNode>(N0.getOperand(0))) {
----------------
asb wrote:

> Hm, actually, I think we might want to exploring doing this even if the op _isn't_ a load. The shifted masks correspond to zext.b/h/w. W has all the uw variants. zext.w is in zba, zext.h is in zbb, zext.b in base, and all the compressed variants in zcb. These are likely either going to fold into something, or if not, be cheaper/smaller than the shifted mask formation.
> 
> (As a possible follow up is totally fine.)

I initially thought there was potential here too and I'd kicked the tires on this a little bit before posting this PR. Applying this transform more generally does allow things like:
```
 slli    a0, a0, 56
 srli    a0, a0, 48
=>
 zext.b  a0, a0
 slli    a0, a0, 8
```
But as Craig says there's no benefit in terms of compressibiity (I did have to double check...).

https://github.com/llvm/llvm-project/pull/170483