[AArch64][SVE] Improve fixed-length addressing modes. (#129732)

When compiling VLS SVE, the compiler often replaces VL-based offsets
with immediate-based ones. This leads to a mismatch in the allowed
addressing modes due to SVE loads/stores generally expecting immediate
offsets relative to VL. For example, given:
```c

svfloat64_t foo(const double *x) {
  svbool_t pg = svptrue_b64();
  return svld1_f64(pg, x+svcntd());
}
```

When compiled with `-msve-vector-bits=128`, we currently generate:
```gas
foo:
        ptrue   p0.d
        mov     x8, #2
        ld1d    { z0.d }, p0/z, [x0, x8, lsl #3]
        ret
```

Instead, we could be generating:
```gas
foo:
        ldr     z0, [x0, #1, mul vl]
        ret
```

Likewise for other types, stores, and other VLS lengths.

This patch achieves the above by extending `SelectAddrModeIndexedSVE`
to let constants through when `vscale` is known.

Author: rj-jesus

clang/test/CodeGen/AArch64/sve-vector-bits-codegen.c

@@ -13,12 +13,9 @@
 
 void func(int *restrict a, int *restrict b) {
 // CHECK-LABEL: func
-// CHECK256-COUNT-1: str
-// CHECK256-COUNT-7: st1w
-// CHECK512-COUNT-1: str
-// CHECK512-COUNT-3: st1w
-// CHECK1024-COUNT-1: str
-// CHECK1024-COUNT-1: st1w
+// CHECK256-COUNT-8: str
+// CHECK512-COUNT-4: str
+// CHECK1024-COUNT-2: str
 // CHECK2048-COUNT-1: st1w
 #pragma clang loop vectorize(enable)
   for (int i = 0; i < 64; ++i)

llvm/lib/Target/AArch64/AArch64ISelDAGToDAG.cpp

@@ -7380,12 +7380,23 @@ bool AArch64DAGToDAGISel::SelectAddrModeIndexedSVE(SDNode *Root, SDValue N,
     return false;
 
   SDValue VScale = N.getOperand(1);
-  if (VScale.getOpcode() != ISD::VSCALE)
+  int64_t MulImm = std::numeric_limits<int64_t>::max();
+  if (VScale.getOpcode() == ISD::VSCALE) {
+    MulImm = cast<ConstantSDNode>(VScale.getOperand(0))->getSExtValue();
+  } else if (auto C = dyn_cast<ConstantSDNode>(VScale)) {
+    int64_t ByteOffset = C->getSExtValue();
+    const auto KnownVScale =
+        Subtarget->getSVEVectorSizeInBits() / AArch64::SVEBitsPerBlock;
+
+    if (!KnownVScale || ByteOffset % KnownVScale != 0)
+      return false;
+
+    MulImm = ByteOffset / KnownVScale;
+  } else
     return false;
 
   TypeSize TS = MemVT.getSizeInBits();
   int64_t MemWidthBytes = static_cast<int64_t>(TS.getKnownMinValue()) / 8;
-  int64_t MulImm = cast<ConstantSDNode>(VScale.getOperand(0))->getSExtValue();
 
   if ((MulImm % MemWidthBytes) != 0)
     return false;

llvm/lib/Target/AArch64/AArch64Subtarget.h

@@ -391,7 +391,7 @@ class AArch64Subtarget final : public AArch64GenSubtargetInfo {
   void mirFileLoaded(MachineFunction &MF) const override;
 
   // Return the known range for the bit length of SVE data registers. A value
-  // of 0 means nothing is known about that particular limit beyong what's
+  // of 0 means nothing is known about that particular limit beyond what's
   // implied by the architecture.
   unsigned getMaxSVEVectorSizeInBits() const {
     assert(isSVEorStreamingSVEAvailable() &&
@@ -405,6 +405,16 @@ class AArch64Subtarget final : public AArch64GenSubtargetInfo {
     return MinSVEVectorSizeInBits;
   }
 
+  // Return the known bit length of SVE data registers. A value of 0 means the
+  // length is unkown beyond what's implied by the architecture.
+  unsigned getSVEVectorSizeInBits() const {
+    assert(isSVEorStreamingSVEAvailable() &&
+           "Tried to get SVE vector length without SVE support!");
+    if (MinSVEVectorSizeInBits == MaxSVEVectorSizeInBits)
+      return MaxSVEVectorSizeInBits;
+    return 0;
+  }
+
   bool useSVEForFixedLengthVectors() const {
     if (!isSVEorStreamingSVEAvailable())
       return false;