Simplifying byteswap(byteswap(a)) to a.

src/alloy/compiler/passes/simplification_pass.cc

@@ -56,6 +56,11 @@ void SimplificationPass::EliminateConversions(HIRBuilder* builder) {
       } else if (i->opcode == &OPCODE_ZERO_EXTEND_info) {
         // Matches truncate + zero_extend.
         CheckZeroExtend(i);
+      } else if (i->opcode == &OPCODE_BYTE_SWAP_info) {
+        // Matches byte swap + byte swap.
+        // This is pretty rare within the same basic block, but is in the
+        // memcpy hot path and (probably) worth it. Maybe.
+        CheckByteSwap(i);
       }
       i = i->next;
     }
@@ -110,6 +115,25 @@ void SimplificationPass::CheckZeroExtend(Instr* i) {
   }
 }
 
+void SimplificationPass::CheckByteSwap(Instr* i) {
+  // Walk backward up src's chain looking for a byte swap. We may have
+  // assigns, so skip those.
+  auto src = i->src1.value;
+  Instr* def = src->def;
+  while (def && def->opcode == &OPCODE_ASSIGN_info) {
+    // Skip asignments.
+    def = def->src1.value->def;
+  }
+  if (def && def->opcode == &OPCODE_BYTE_SWAP_info) {
+    // Value comes from a byte swap.
+    if (def->src1.value->type == i->dest->type) {
+      // Types match, use original by turning this into an assign.
+      i->Replace(&OPCODE_ASSIGN_info, 0);
+      i->set_src1(def->src1.value);
+    }
+  }
+}
+
 void SimplificationPass::SimplifyAssignments(HIRBuilder* builder) {
   // Run over the instructions and rename assigned variables:
   //   v1 = v0

src/alloy/compiler/passes/simplification_pass.h

@@ -29,6 +29,7 @@ private:
   void EliminateConversions(hir::HIRBuilder* builder);
   void CheckTruncate(hir::Instr* i);
   void CheckZeroExtend(hir::Instr* i);
+  void CheckByteSwap(hir::Instr* i);
 
   void SimplifyAssignments(hir::HIRBuilder* builder);
   hir::Value* CheckValue(hir::Value* value);