[RISCV] Implement RISCVTTIImpl::getPreferredAddressingMode for HasVendorXCVmem (#120533)

For a simple matmult kernel this heuristic reduces the length of the
critical basic block from 15 to 20 instructions, resulting in a 20%
speedup.

**Without heuristic:**

```
       13688: 001b838b      cv.lb   t2, (s7), 0x1
       1368c: 09cdbcab      cv.lb   s9, t3(s11)
       13690: 089db62b      cv.lb   a2, s1(s11)
       13694: 092dbdab      cv.lb   s11, s2(s11)
       13698: 001d028b      cv.lb   t0, (s10), 0x1
       1369c: 00f282b3      add     t0, t0, a5
       136a0: 9072b52b      cv.mac  a0, t0, t2
       136a4: 9192bfab      cv.mac  t6, t0, s9
       136a8: 90c2beab      cv.mac  t4, t0, a2
       136ac: 91b2bf2b      cv.mac  t5, t0, s11
       136b0: fffc0c13      addi    s8, s8, -0x1
       136b4: 018e0633      add     a2, t3, s8
       136b8: 91b2b0ab      cv.mac  ra, t0, s11
       136bc: 000b8d93      mv      s11, s7
       136c0: fc0614e3      bnez    a2, 0x13688 <muriscv_nn_vec_mat_mult_t_s8+0x2f0>

       #instrs = 15
```

**With heuristic:**

```
        7bc0: 001c860b      cv.lb   a2, (s9), 0x1
        7bc4: 001e0d0b      cv.lb   s10, (t3), 0x1
        7bc8: 001e808b      cv.lb   ra, (t4), 0x1
        7bcc: 0015038b      cv.lb   t2, (a0), 0x1
        7bd0: 001c028b      cv.lb   t0, (s8), 0x1
        7bd4: 00f282b3      add     t0, t0, a5
        7bd8: 90c2bfab      cv.mac  t6, t0, a2
        7bdc: 91a2b92b      cv.mac  s2, t0, s10
        7be0: 9012b5ab      cv.mac  a1, t0, ra
        7be4: 9072b9ab      cv.mac  s3, t0, t2
        7be8: 9072b72b      cv.mac  a4, t0, t2
        7bec: fc851ae3      bne     a0, s0, 0x7bc0 <muriscv_nn_vec_mat_mult_t_s8+0x338>

        #instrs = 12

        improvement = 1 - 12/15 = 0.2 = 20%
```

Author: PhilippvK

llvm/lib/Target/RISCV/RISCVTargetTransformInfo.cpp

@@ -2329,6 +2329,15 @@ unsigned RISCVTTIImpl::getMaximumVF(unsigned ElemWidth, unsigned Opcode) const {
   return std::max<unsigned>(1U, RegWidth.getFixedValue() / ElemWidth);
 }
 
+TTI::AddressingModeKind
+RISCVTTIImpl::getPreferredAddressingMode(const Loop *L,
+                                         ScalarEvolution *SE) const {
+  if (ST->hasVendorXCVmem() && !ST->is64Bit())
+    return TTI::AMK_PostIndexed;
+
+  return BasicTTIImplBase::getPreferredAddressingMode(L, SE);
+}
+
 bool RISCVTTIImpl::isLSRCostLess(const TargetTransformInfo::LSRCost &C1,
                                  const TargetTransformInfo::LSRCost &C2) {
   // RISC-V specific here are "instruction number 1st priority".

llvm/lib/Target/RISCV/RISCVTargetTransformInfo.h

@@ -388,6 +388,9 @@ class RISCVTTIImpl : public BasicTTIImplBase<RISCVTTIImpl> {
     llvm_unreachable("unknown register class");
   }
 
+  TTI::AddressingModeKind getPreferredAddressingMode(const Loop *L,
+                                                     ScalarEvolution *SE) const;
+
   unsigned getRegisterClassForType(bool Vector, Type *Ty = nullptr) const {
     if (Vector)
       return RISCVRegisterClass::VRRC;

llvm/test/CodeGen/RISCV/xcvmem-heuristic.ll

@@ -0,0 +1,34 @@
+; NOTE: Assertions have been autogenerated by utils/update_llc_test_checks.py
+; RUN: llc -O3 -mtriple=riscv32 -mattr=+m,+xcvmem -verify-machineinstrs < %s \
+; RUN:   | FileCheck %s --check-prefixes=CHECK
+
+define i32 @test_heuristic(ptr %b, i32 %e, i1 %0) {
+; CHECK-LABEL: test_heuristic:
+; CHECK:       # %bb.0: # %entry
+; CHECK-NEXT:    add a3, a0, a1
+; CHECK-NEXT:    andi a2, a2, 1
+; CHECK-NEXT:  .LBB0_1: # %loop
+; CHECK-NEXT:    # =>This Inner Loop Header: Depth=1
+; CHECK-NEXT:    cv.lbu a1, (a3), 1
+; CHECK-NEXT:    addi a0, a0, 1
+; CHECK-NEXT:    beqz a2, .LBB0_1
+; CHECK-NEXT:  # %bb.2: # %exit
+; CHECK-NEXT:    mv a0, a1
+; CHECK-NEXT:    ret
+entry:
+  %1 = getelementptr i8, ptr %b, i32 %e
+  br label %loop
+
+loop:                                             ; preds = %loop, %entry
+  %2 = phi ptr [ %b, %entry ], [ %7, %loop ]
+  %3 = phi ptr [ %1, %entry ], [ %8, %loop ]
+  %4 = load i8, ptr %2, align 1
+  %5 = load i8, ptr %3, align 1
+  %6 = zext i8 %5 to i32
+  %7 = getelementptr i8, ptr %2, i32 1
+  %8 = getelementptr i8, ptr %3, i32 1
+  br i1 %0, label %exit, label %loop
+
+exit:                                             ; preds = %loop
+  ret i32 %6
+}