[llvm] [AArch64][CodeGen] Add patterns for small negative VScale const (PR #89607)

[Paul Walker via llvm-commits]

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@@ -2583,6 +2583,27 @@ let Predicates = [HasSVEorSME] in {
                                sub_32)>;
   }
 
+  // Add NoUseScalarIncVL to avoid affecting for patterns with UseScalarIncVL
+  let Predicates = [NoUseScalarIncVL] in {
+    def : Pat<(add GPR64:$op, (vscale (sve_rdvl_imm i32:$imm))),
+              (ADDXrs GPR64:$op, (RDVLI_XI $imm), 0)>;
+    def : Pat<(add GPR64:$op, (vscale (sve_cnth_imm_neg i32:$imm))),
+              (SUBXrs GPR64:$op, (CNTH_XPiI 31, $imm), 0)>;
+    def : Pat<(add GPR64:$op, (vscale (sve_cntw_imm_neg i32:$imm))),
+              (SUBXrs GPR64:$op, (CNTW_XPiI 31, $imm), 0)>;
+    def : Pat<(add GPR64:$op, (vscale (sve_cntd_imm_neg i32:$imm))),
+              (SUBXrs GPR64:$op, (CNTD_XPiI 31, $imm), 0)>;
+
+    def : Pat<(add GPR32:$op, (i32 (trunc (vscale (sve_rdvl_imm i32:$imm))))),
+              (ADDSWrr GPR32:$op, (EXTRACT_SUBREG (RDVLI_XI $imm), sub_32))>;
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paulwalker-arm wrote:

As above, I don't think we need to worry about the cases where `sve_rdvl_imm` already match.

https://github.com/llvm/llvm-project/pull/89607