Merge pull request #13 from yyyc514/sprites_assembly_buffer_underflow

Fix buffer underflow in drawPlusMask

src/Sprites.cpp

@@ -285,19 +285,23 @@ void Sprites::drawBitmap(int16_t x, int16_t y,
 
 
         // FIRST PAGE
-        "ld %[data], %a[buffer_ofs]\n"
         // if sRow >= 0
         "tst %[sRow]\n"
-        "brmi end_first_page\n"
+        "brmi skip_first_page\n"
+        "ld %[data], %a[buffer_ofs]\n"
         // then
         "com %A[mask_data]\n"
         "and %[data], %A[mask_data]\n"
         "or %[data], %A[bitmap_data]\n"
-
-        "end_first_page:\n"
         // update buffer, increment
         "st %a[buffer_ofs]+, %[data]\n"
+        "jmp end_first_page\n"
 
+        "skip_first_page:\n"
+        // since no ST Z+ when skipped we need to do this manually
+        "adiw %[buffer_ofs], 1\n"
+
+        "end_first_page:\n"
 
         // "x_loop_next:\n"
         "dec %[xi]\n"
@@ -330,9 +334,9 @@ void Sprites::drawBitmap(int16_t x, int16_t y,
         : [xi] "+&r" (xi),
         [yi] "+&r" (yi),
         [sRow] "+&d" (sRow), // CPI requires an upper register
-        [data] "+&r" (data),
-        [mask_data] "+&r" (mask_data),
-        [bitmap_data] "+&r" (bitmap_data)
+        [data] "=&r" (data),
+        [mask_data] "=&r" (mask_data),
+        [bitmap_data] "=&r" (bitmap_data)
         :
         [x_count] "r" (rendered_width),
         [y_count] "r" (loop_h),
@@ -343,7 +347,10 @@ void Sprites::drawBitmap(int16_t x, int16_t y,
         [sprite_ofs_jump] "r" ((w-rendered_width)*2),
         [yOffset] "r" (yOffset),
         [mul_amt] "r" (mul_amt)
-        :
+        // declaring an extra high register clobber here for some reason
+        // prevents a compile error for some sketches:
+        // can't find a register in class 'LD_REGS' while reloading 'asm'
+        : "r24"
       );
       break;
   }