agx: add pseudo for signext

easier to optimize

Signed-off-by: Alyssa Rosenzweig <alyssa@rosenzweig.io>
Part-of: <https://gitlab.freedesktop.org/mesa/mesa/-/merge_requests/32081>

src/asahi/compiler/agx_compile.c

@@ -1982,7 +1982,7 @@ agx_emit_alu(agx_builder *b, nir_alu_instr *instr)
          return agx_asr_to(b, dst, ishl16, agx_immediate(8));
       } else {
          assert(s0.size == AGX_SIZE_16 && "other conversions lowered");
-         return agx_iadd_to(b, dst, s0, i0, 0);
+         return agx_signext_to(b, dst, s0);
       }
    }
 

src/asahi/compiler/agx_lower_pseudo.c

@@ -83,6 +83,10 @@ lower(agx_builder *b, agx_instr *I)
    case AGX_OPCODE_NOT:
       return agx_bitop_to(b, I->dest[0], I->src[0], agx_zero(), AGX_BITOP_NOT);
 
+   /* We can sign-extend with an add */
+   case AGX_OPCODE_SIGNEXT:
+      return agx_iadd_to(b, I->dest[0], I->src[0], agx_zero(), 0);
+
    /* Unfused comparisons are fused with a 0/1 select */
    case AGX_OPCODE_ICMP:
       return agx_icmpsel_to(b, I->dest[0], I->src[0], I->src[1],

src/asahi/compiler/agx_opcodes.py

@@ -501,6 +501,7 @@ op("stack_store",
 # Convenient aliases.
 op("mov", _, srcs = 1)
 op("not", _, srcs = 1)
+op("signext", _, srcs = 1)
 
 op("collect", _, srcs = VARIABLE)
 op("split", _, srcs = 1, dests = VARIABLE)