[AMDGPU] Try decoding instructions longest first. NFCI. (#82014)

AMDGPUDisassembler::getInstruction tries decoding instructions using
different DecoderTables in a confusing order: first 96-bit instructions,
then some 64-bit, then 32-bit, then some more 64-bit.

This patch changes it to always try longer encodings first. The
motivation is to make getInstruction easier to understand, and to pave
the way for combining some 64-bit tables that do not need to be
separate.

Author: jayfoad

llvm/lib/Target/AMDGPU/Disassembler/AMDGPUDisassembler.cpp

@@ -595,6 +595,48 @@ DecodeStatus AMDGPUDisassembler::getInstruction(MCInst &MI, uint64_t &Size,
         if (Res)
           break;
       }
+
+      if (STI.hasFeature(AMDGPU::FeatureGFX940Insts)) {
+        Res = tryDecodeInst(DecoderTableGFX94064, MI, QW, Address, CS);
+        if (Res)
+          break;
+      }
+
+      if (STI.hasFeature(AMDGPU::FeatureGFX90AInsts)) {
+        Res = tryDecodeInst(DecoderTableGFX90A64, MI, QW, Address, CS);
+        if (Res)
+          break;
+      }
+
+      Res = tryDecodeInst(DecoderTableGFX864, MI, QW, Address, CS);
+      if (Res)
+        break;
+
+      Res = tryDecodeInst(DecoderTableGFX964, MI, QW, Address, CS);
+      if (Res)
+        break;
+
+      Res = tryDecodeInst(DecoderTableGFX1064, MI, QW, Address, CS);
+      if (Res)
+        break;
+
+      Res = tryDecodeInst(DecoderTableGFX1264, DecoderTableGFX12_FAKE1664, MI,
+                          QW, Address, CS);
+      if (Res)
+        break;
+
+      Res = tryDecodeInst(DecoderTableGFX1164, DecoderTableGFX11_FAKE1664, MI,
+                          QW, Address, CS);
+      if (Res)
+        break;
+
+      Res = tryDecodeInst(DecoderTableGFX11W6464, MI, QW, Address, CS);
+      if (Res)
+        break;
+
+      Res = tryDecodeInst(DecoderTableGFX12W6464, MI, QW, Address, CS);
+      if (Res)
+        break;
     }
 
     // Reinitialize Bytes as DPP64 could have eaten too much
@@ -632,48 +674,6 @@ DecodeStatus AMDGPUDisassembler::getInstruction(MCInst &MI, uint64_t &Size,
 
     Res = tryDecodeInst(DecoderTableGFX1232, DecoderTableGFX12_FAKE1632, MI, DW,
                         Address, CS);
-    if (Res)
-      break;
-
-    if (Bytes.size() < 4) break;
-    const uint64_t QW = ((uint64_t)eatBytes<uint32_t>(Bytes) << 32) | DW;
-
-    if (STI.hasFeature(AMDGPU::FeatureGFX940Insts)) {
-      Res = tryDecodeInst(DecoderTableGFX94064, MI, QW, Address, CS);
-      if (Res)
-        break;
-    }
-
-    if (STI.hasFeature(AMDGPU::FeatureGFX90AInsts)) {
-      Res = tryDecodeInst(DecoderTableGFX90A64, MI, QW, Address, CS);
-      if (Res)
-        break;
-    }
-
-    Res = tryDecodeInst(DecoderTableGFX864, MI, QW, Address, CS);
-    if (Res) break;
-
-    Res = tryDecodeInst(DecoderTableGFX964, MI, QW, Address, CS);
-    if (Res) break;
-
-    Res = tryDecodeInst(DecoderTableGFX1064, MI, QW, Address, CS);
-    if (Res) break;
-
-    Res = tryDecodeInst(DecoderTableGFX1264, DecoderTableGFX12_FAKE1664, MI, QW,
-                        Address, CS);
-    if (Res)
-      break;
-
-    Res = tryDecodeInst(DecoderTableGFX1164, DecoderTableGFX11_FAKE1664, MI, QW,
-                        Address, CS);
-    if (Res)
-      break;
-
-    Res = tryDecodeInst(DecoderTableGFX11W6464, MI, QW, Address, CS);
-    if (Res)
-      break;
-
-    Res = tryDecodeInst(DecoderTableGFX12W6464, MI, QW, Address, CS);
   } while (false);
 
   if (Res && AMDGPU::isMAC(MI.getOpcode())) {

llvm/lib/Target/AMDGPU/SOPInstructions.td

@@ -2571,11 +2571,13 @@ multiclass SOPP_Real_32_gfx11_Renamed_gfx12<bits<7> op, string gfx12_name> :
 
 multiclass SOPP_Real_With_Relaxation_gfx12<bits<7> op> {
   defm "" : SOPP_Real_32_gfx12<op>;
+  let isCodeGenOnly = 1 in
   defm _pad_s_nop : SOPP_Real_64_gfx12<op>;
 }
 
 multiclass SOPP_Real_With_Relaxation_gfx11<bits<7> op> {
   defm "" : SOPP_Real_32_gfx11<op>;
+  let isCodeGenOnly = 1 in
   defm _pad_s_nop : SOPP_Real_64_gfx11<op>;
 }
 
@@ -2697,6 +2699,7 @@ multiclass SOPP_Real_64_gfx6_gfx7_gfx8_gfx9_gfx10<bits<7> op> :
 //relaxation for insts with no operands not implemented
 multiclass SOPP_Real_With_Relaxation_gfx6_gfx7_gfx8_gfx9_gfx10<bits<7> op> {
   defm "" : SOPP_Real_32_gfx6_gfx7_gfx8_gfx9_gfx10<op>;
+  let isCodeGenOnly = 1 in
   defm _pad_s_nop : SOPP_Real_64_gfx6_gfx7_gfx8_gfx9_gfx10<op>;
 }