[llvm] [RISCV][VLOPT] Support segmented store instructions (PR #155467)

[Luke Lau via llvm-commits]

================
@@ -2192,3 +2192,219 @@ body: |
     %x:vr = PseudoVADD_VV_M1 $noreg, $noreg, $noreg, -1, 4 /* e16 */, 0
     %y:vr = PseudoVRGATHEREI16_VV_M1_E32_MF2 $noreg, $noreg, %x, 1, 5 /* e32 */, 0
     $v8 = COPY %y
+...
+---
+name: vsseg3e32_v
+body: |
+  bb.0:
+    liveins: $v8
+
+    ; CHECK-LABEL: name: vsseg3e32_v
+    ; CHECK: liveins: $v8
+    ; CHECK-NEXT: {{  $}}
+    ; CHECK-NEXT: [[COPY:%[0-9]+]]:vr = COPY $v8
+    ; CHECK-NEXT: [[PseudoVADD_VV_M1_:%[0-9]+]]:vr = PseudoVADD_VV_M1 $noreg, $noreg, $noreg, 1, 5 /* e32 */, 3 /* ta, ma */
+    ; CHECK-NEXT: [[PseudoVADD_VV_M1_1:%[0-9]+]]:vr = PseudoVADD_VV_M1 $noreg, $noreg, $noreg, 1, 5 /* e32 */, 3 /* ta, ma */
+    ; CHECK-NEXT: [[DEF:%[0-9]+]]:vrn3m1 = IMPLICIT_DEF
+    ; CHECK-NEXT: [[INSERT_SUBREG:%[0-9]+]]:vrn3m1 = INSERT_SUBREG [[DEF]], [[COPY]], %subreg.sub_vrm1_0
+    ; CHECK-NEXT: [[INSERT_SUBREG1:%[0-9]+]]:vrn3m1 = INSERT_SUBREG [[INSERT_SUBREG]], [[PseudoVADD_VV_M1_]], %subreg.sub_vrm1_1
+    ; CHECK-NEXT: [[INSERT_SUBREG2:%[0-9]+]]:vrn3m1 = INSERT_SUBREG [[INSERT_SUBREG1]], [[PseudoVADD_VV_M1_1]], %subreg.sub_vrm1_2
+    ; CHECK-NEXT: PseudoVSSEG3E32_V_M1 killed [[INSERT_SUBREG2]], $noreg, 1, 5 /* e32 */
+    %0:vr = COPY $v8
+    %1:vr = PseudoVADD_VV_M1 $noreg, $noreg, $noreg, 10, 5 /* e32 */, 3 /* ta, ma */
+    %2:vr = PseudoVADD_VV_M1 $noreg, $noreg, $noreg, -1, 5 /* e32 */, 3 /* ta, ma */
+    %6:vrn3m1 = IMPLICIT_DEF
+    %5:vrn3m1 = INSERT_SUBREG %6, %0, %subreg.sub_vrm1_0
+    %7:vrn3m1 = INSERT_SUBREG %5, %1, %subreg.sub_vrm1_1
+    %8:vrn3m1 = INSERT_SUBREG %7, %2, %subreg.sub_vrm1_2
+    PseudoVSSEG3E32_V_M1 killed %8, $noreg, 1, 5 /* e32 */
----------------
lukel97 wrote:

Are we always guaranteed that every user of an insert_subreg into a vrnXm1 register is a segmented store? 

E.g. is it possible via bitcasts that we might ever get something like

```
%8:vrn4m1 = INSERT_SUBREG %7, %2, %subreg.sub_vrm1_2
%9:vrm4 = COPY %8
PseudoVSE32_V_M1 %9:vrm4, %ptr, 1, 5 /* e32 */
```

Which in this case I don't think we can propagate the VL from the vse because it's not "per-segment". Is this something we can assert at the start of the pass?

https://github.com/llvm/llvm-project/pull/155467