|
| Author |
Message |
|
hudson
|
Post subject: Inline constants for colors  Posted: Sat Mar 26, 2011 4:34 pm |
Joined: Sun Feb 27, 2011 11:32 am
Posts: 71
|
Since memory is at a premium, it saves a few bytes to use C99 inline struct constants for colors instead of static const variables: --- include/pulse_types.h-orig 2011-03-26 20:41:27.000000000 -0400
+++ include/pulse_types.h 2011-03-26 20:41:41.000000000 -0400
@@ -12,8 +12,8 @@
} __attribute__((packed)) color24_t;
/* Some handy colors to use. */
-static const color24_t COLOR_BLACK24 = {0, 0, 0, 0};
-static const color24_t COLOR_WHITE24 = {0xff, 0xff, 0xff, 0};
+#define COLOR_BLACK24 ((color24_t){0,0,0,0})
+#define COLOR_WHITE24 ((color24_t){255,255,255,0})
// This struct is almost identical to a struct tm time struct (in C), but with some key differences.
In my single file test case this saved 16 bytes (8 bytes for the data and 8 bytes of loading instructions replaced with a single load immediate); if the constants are used in more than one file or more than once per file, then the savings can be even larger. There are some cases, however, where colors that are more complex than all zero or all ones might require more instructions to build immediate values. But white and black are definitely good candidates for inlining.
|
|
|
|
 |
|
hudson
|
Post subject: Re: Inline constants for colors Posted: Sat Mar 26, 2011 4:45 pm |
Joined: Sun Feb 27, 2011 11:32 am
Posts: 71
|
The actual code generated is pretty crazy: #include <pulse_types.h>
extern void foo(color24_t c);
void const_color(void)
{
foo(COLOR_BLACK24);
}
void inline_color(void)
{
foo((color24_t){0,0,0,0});
}
Produces the following functions, for a savings of 16 bytes of code + 4 bytes of data:
00000000 <inline_color>:
0: e3a03000 mov r3, #0
4: e24dd004 sub sp, sp, #4
8: e1a00003 mov r0, r3
c: e58d3000 str r3, [sp]
10: e28dd004 add sp, sp, #4
14: eafffffe b 0 <foo>
versus: 00000000 <const_color>:
0: e59f301c ldr r3, [pc, #28] ; 24 <const_color+0x24>
4: e5d30000 ldrb r0, [r3]
8: e5d32001 ldrb r2, [r3, #1]
c: e5d31002 ldrb r1, [r3, #2]
10: e1800402 orr r0, r0, r2, lsl #8
14: e5d33003 ldrb r3, [r3, #3]
18: e1800801 orr r0, r0, r1, lsl #16
1c: e1800c03 orr r0, r0, r3, lsl #24
20: eafffffe b 0 <foo>
24: 00000000 andeq r0, r0, r0
Disassembly of section .rodata:
00000000 <COLOR_BLACK24>:
0: 00000000 andeq r0, r0, r0
|
|
|
|
|
|
|
|
