Imagine you want to define a C preprocessor statement like this:<\/p>\n
#define NUM_NOPS(i) .....<\/pre>\nand later on, in your code:<\/p>\n
NUM_NOPS(5);<\/pre>\nand you want your preprocessor to roll out the following code:<\/p>\n
\r\nasm volatile("nop");\r\nasm volatile("nop");\r\nasm volatile("nop");\r\nasm volatile("nop");\r\nasm volatile("nop");\r\n<\/pre>\nThis is not as easy as it looks like. One possibility is to use Boost’s Preprocessor Repetition Utility: <boost\/preprocessor\/repetition\/repeat.hpp><\/em>. But however, let’s assume you don’t want to use any third party libraries. There is one further possibility. You can use GCC’s unrool loops functionality:<\/p>\n
\r\n#pragma GCC push_options\r\n#pragma GCC optimize ("unroll-loops")\r\n#pragma GCC optimize ("O2")\r\nvoid nops() {\r\n int i = 0;\r\n for(; i < 10 ; i++) {\r\n asm volatile("nop");\r\n }\r\n}\r\n#pragma GCC pop_options\r\n\r\nint main(void) {\r\n nops();\r\n}\r\n<\/pre>\nYou should compile it with:<\/p>\n
gcc -c -o test.o -O3 test.c <\/p>\n
Having a look at the disassembly proves, that your loop was rolled out:<\/p>\n
\r\nobjdump -d test.o\r\n\r\ntest.o: file format elf64-x86-64\r\n\r\n\r\nDisassembly of section .text:\r\n\r\n0000000000000000 <nops>:\r\n 0: 90 nop\r\n 1: 90 nop\r\n 2: 90 nop\r\n 3: 90 nop\r\n 4: 90 nop\r\n 5: 90 nop\r\n 6: 90 nop\r\n 7: 90 nop\r\n 8: 90 nop\r\n 9: 90 nop\r\n a: c3 retq \r\n\r\nDisassembly of section .text.startup:\r\n\r\n0000000000000000 <main>:\r\n 0: 31 c0 xor %eax,%eax\r\n 2: e8 00 00 00 00 callq 7 <main+0x7>\r\n 7: f3 c3 repz retq \r\n<\/pre>\n","protected":false},"excerpt":{"rendered":"Imagine you want to define a C preprocessor statement like this: #define NUM_NOPS(i) ….. and later on, in your code: NUM_NOPS(5); and you want your preprocessor to roll out the following code: asm volatile("nop"); asm volatile("nop"); asm volatile("nop"); asm volatile("nop"); asm volatile("nop"); This is not as easy as it looks like. One possibility is to … Continue reading Self Expanding Loops in C<\/span>