[llvm] [RISCV][VLOPT] Compute demanded VLs up front. NFC (PR #124530)

[Luke Lau via llvm-commits]

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@@ -12,9 +12,11 @@ body: |
     ; CHECK-NEXT: %vl:gprnox0 = COPY $x1
     ; CHECK-NEXT: %x:vr = PseudoVADD_VV_MF4 $noreg, $noreg, $noreg, -1, 4 /* e16 */, 0 /* tu, mu */
     ; CHECK-NEXT: %y:vr = PseudoVNSRL_WV_MF4 $noreg, %x, $noreg, %vl, 4 /* e16 */, 0 /* tu, mu */
+    ; CHECK-NEXT: $v8 = COPY %y
     %vl:gprnox0 = COPY $x1
     %x:vr = PseudoVADD_VV_MF4 $noreg, $noreg, $noreg, -1, 4 /* e16 */, 0 /* tu, mu */
     %y:vr = PseudoVNSRL_WV_MF4 $noreg, %x, $noreg, %vl, 4 /* e16 */, 0 /* tu, mu */
+    $v8 = COPY %y
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lukel97 wrote:

Because we're now checking what elements are demanded, any instruction without a use will have a VL of zero demanded, which will end up reducing everything else in the basic block to VL=0. I've added COPYs in the .mir tests to prevent this

https://github.com/llvm/llvm-project/pull/124530