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

[Philipp van Kempen via llvm-commits]

PhilippvK wrote:

> Can you add a test?

@wangpc-pp I looked for existing tests which perform a similar check but was not able to find anything useful. Hence I've tried to come up with a "minimal" example which shows that the heuristic is being applied.

I am not very happy with it and am happy about any feedback on how to make the test more compact / less flaky:

```ll
; 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 %a, ptr %b, i32 %c, i32 %d, i32 %e) nounwind {
; CHECK-LABEL: test_heuristic:
; CHECK:  # %bb.0:                                # %entry
; CHECK-NEXT:          beqz    a0, .LBB0_4
; CHECK-NEXT:  # %bb.1:                                # %prepare
; CHECK-NEXT:          add     a5, a1, a4
; CHECK-NEXT:          add     a3, a4, a3
; CHECK-NEXT:          add     a1, a1, a3
; CHECK-NEXT:  .LBB0_2:                                # %loop
; CHECK-NEXT:                                          # =>This Inner Loop Header: Depth=1
; CHECK-NEXT:          cv.lbu  a3, (a5), 1
; CHECK-NEXT:          cv.lbu  a4, (a0), 1
; CHECK-NEXT:          bne     a5, a1, .LBB0_2
; CHECK-NEXT:  # %bb.3:
; CHECK-NEXT:          add     a2, a4, a2
; CHECK-NEXT:          mul     a0, a2, a3
; CHECK-NEXT:  .LBB0_4:                                # %exit
; CHECK-NEXT:          ret
entry:
  %4 = icmp eq ptr %a, null
  br i1 %4, label %exit, label %prepare

prepare:
  %20 = shl nsw i32 %e, 2
  %21 = mul nuw nsw i32 %e, 3
  %22 = shl nuw nsw i32 %e, 1
  %23 = getelementptr inbounds i8, ptr %b, i32 %e
  %24 = getelementptr inbounds i8, ptr %b, i32 %22
  %25 = getelementptr inbounds i8, ptr %b, i32 %21
  %26 = getelementptr inbounds i8, ptr %b, i32 %20
  br label %loop

loop:
  %28 = phi ptr [ %a, %prepare ], [ %68, %loop ]
  %29 = phi ptr [ %b, %prepare ], [ %63, %loop ]
  %30 = phi ptr [ %23, %prepare ], [ %64, %loop ]
  %32 = phi i32 [ 0, %prepare ], [ %69, %loop ]
  %37 = phi i32 [ 0, %prepare ], [ %54, %loop ]
  %40 = load i8, ptr %29, align 1
  %41 = zext i8 %40 to i32
  %42 = load i8, ptr %30, align 1
  %43 = zext i8 %42 to i32
  %50 = load i8, ptr %28, align 1
  %51 = zext i8 %50 to i32
  %52 = add i32 %51, %c
  %53 = mul i32 %52, %41
  %54 = add i32 %53, %37
  %55 = mul i32 %52, %43
  %63 = getelementptr inbounds i8, ptr %29, i32 1
  %64 = getelementptr inbounds i8, ptr %30, i32 1
  %68 = getelementptr inbounds i8, ptr %28, i32 1
  %69 = add nuw nsw i32 %32, 1
  %70 = icmp eq i32 %69, %d
  br i1 %70, label %exit, label %loop

exit:
  %result = phi i32 [ 0, %entry ], [ %55, %loop ]
  ret i32 %result
}
```

https://github.com/llvm/llvm-project/pull/120533