6760

Fix a couple of subtle bugs.

- the VM was conditionally reading from the instruction stream, so that
  other bugs got masked by decoding errors.
- push-n-bytes was clobbering eax.

014indirect_addressing.cc

@@ -936,7 +936,7 @@ void test_add_r32_to_mem_at_r32_plus_disp32() {
 }
 
 :(before "End Mod Special-cases(addr)")
-case 2:  // indirect + disp32 addressing
+case 2: {  // indirect + disp32 addressing
   switch (rm) {
   default:
     addr = Reg[rm].u;
@@ -944,11 +944,16 @@ case 2:  // indirect + disp32 addressing
     break;
   // End Mod 2 Special-cases(addr)
   }
+  int32_t displacement = static_cast<int32_t>(next32());
   if (addr > 0) {
-    addr += next32();
+    addr += displacement;
     trace(Callstack_depth+1, "run") << "effective address is 0x" << HEXWORD << addr << " (after adding disp32)" << end();
   }
+  else {
+    trace(Callstack_depth+1, "run") << "null address; skipping displacement" << end();
+  }
   break;
+}
 
 :(code)
 void test_add_r32_to_mem_at_r32_plus_negative_disp32() {

302stack_allocate.subx

@@ -37,15 +37,12 @@ $push-n-zero-bytes:prologue:
     89/<- %ebp 4/r32/esp
 $push-n-zero-bytes:copy-ra:
     # -- esp = ebp
-    50/push-eax
-    # -- esp+8 = ebp+4
-    # -- esp+4 = ebp
-    8b/-> *(esp+4) 0/r32/eax
+    89/<- *Push-n-zero-bytes-eax 0/r32/eax
+    8b/-> *esp 0/r32/eax
     2b/subtract *(ebp+4) 4/r32/esp
-    # -- esp+4+n = ebp
-    89/<- *(esp+4) 0/r32/eax
-    58/pop-to-eax
     # -- esp+n = ebp
+    89/<- *esp 0/r32/eax
+    8b/-> *Push-n-zero-bytes-eax 0/r32/eax
 $push-n-zero-bytes:bulk-cleaning:
     89/<- *Push-n-zero-bytes-esp 4/r32/esp
     81 0/subop/add *Push-n-zero-bytes-esp 4/imm32
@@ -60,3 +57,5 @@ Push-n-zero-bytes-ebp:  # (addr int)
   0/imm32
 Push-n-zero-bytes-esp:  # (addr int)
   0/imm32
+Push-n-zero-bytes-eax:
+  0/imm32