3 ms·

When I compile it with GCC 12, this machine code results: 1129: f3 0f 1e fa endbr64 112d: 55 push rbp

by msla 2y ago

When I compile it with GCC 12, this machine code results: 1129: f3 0f 1e fa endbr64 112d: 55 push rbp 112e: 48 89 e5 mov rbp,rsp 1131: b8 01 00 00 00 mov eax,0x1 1136: bf 01 00 00 00 mov edi,0x1 113b: 48 8d 05 c2 0e 00 00 lea rax,[rip+0xec2] # 2004 <_IO_stdin_used+0x4> 1142: 48 89 c6 mov rsi,rax 1145: ba 0f 00 00 00 mov edx,0xf 114a: 0f 05 syscall 114c: b8 00 00 00 00 mov eax,0x0 1151: 5d pop rbp 1152: c3 ret Can you spot the error? . . . . . . The code biffs rax when it loads the string address, so the system call number is lost, and the code ends up not printing anything. Moving the string assignment to be the very first line in main fixes it. BTW, Clang 14 with no optimization accepts the code without issue but compiles it without using any of the registers; it just stores the values to memory locations and runs the syscall opcode. With O1 optimization or higher, it optimizes away everything except the syscall opcode.

rep_lodsb 2y ago
With an older version, it works (as long as there is no optimization at least, with -O2 all the register init code disappears): $ gcc -v ... gcc version 10.2.1 20210110 (Debian 10.2.1-6) 0000000000001125 <main>: 1125: 55 push %rbp 1126: 48 89 e5 mov %rsp,%rbp 1129: b8 01 00 00 00 mov $0x1,%eax 112e: bf 01 00 00 00 mov $0x1,%edi 1133: 48 8d 35 ca 0e 00 00 lea 0xeca(%rip),%rsi # 2004 <_IO_stdin_used+0x4> 113a: ba 0e 00 00 00 mov $0xe,%edx 113f: 0f 05 syscall No idea why a newer version produces worse code in this case (though of course, this way of doing inline assembly isn't "correct" anyway, so nasal demons may result)
khrbtxyz 2y ago
The exact same thing happens with GCC 12 with 32-bit MIPS. #include <asm/unistd.h> char msg[] = "hello, world!\n"; int main(void) { register int syscall_no asm("v0") = __NR_write; register int arg1 asm("a0") = 1; register char *arg2 asm("a1") = msg; register int arg3 asm("a2") = sizeof(msg) - 1; asm("syscall"); return 0; } root@OpenWrt:~# objdump --disassemble=main ... 00400580 <main>: 400580: 27bdfff8 addiu sp,sp,-8 400584: afbe0004 sw s8,4(sp) 400588: 03a0f025 move s8,sp 40058c: 24020fa4 li v0,4004 400590: 24040001 li a0,1 400594: 3c020041 lui v0,0x41 400598: 24450650 addiu a1,v0,1616 40059c: 2406000e li a2,14 4005a0: 0000000c syscall

by msla 2y ago

rep_lodsb 2y ago

With an older version, it works (as long as there is no optimization at least, with -O2 all the register init code disappears): $ gcc -v ... gcc version 10.2.1 20210110 (Debian 10.2.1-6) 0000000000001125 <main>: 1125: 55 push %rbp 1126: 48 89 e5 mov %rsp,%rbp 1129: b8 01 00 00 00 mov $0x1,%eax 112e: bf 01 00 00 00 mov $0x1,%edi 1133: 48 8d 35 ca 0e 00 00 lea 0xeca(%rip),%rsi # 2004 <_IO_stdin_used+0x4> 113a: ba 0e 00 00 00 mov $0xe,%edx 113f: 0f 05 syscall No idea why a newer version produces worse code in this case (though of course, this way of doing inline assembly isn't "correct" anyway, so nasal demons may result)

khrbtxyz 2y ago

The exact same thing happens with GCC 12 with 32-bit MIPS. #include <asm/unistd.h> char msg[] = "hello, world!\n"; int main(void) { register int syscall_no asm("v0") = __NR_write; register int arg1 asm("a0") = 1; register char *arg2 asm("a1") = msg; register int arg3 asm("a2") = sizeof(msg) - 1; asm("syscall"); return 0; } root@OpenWrt:~# objdump --disassemble=main ... 00400580 <main>: 400580: 27bdfff8 addiu sp,sp,-8 400584: afbe0004 sw s8,4(sp) 400588: 03a0f025 move s8,sp 40058c: 24020fa4 li v0,4004 400590: 24040001 li a0,1 400594: 3c020041 lui v0,0x41 400598: 24450650 addiu a1,v0,1616 40059c: 2406000e li a2,14 4005a0: 0000000c syscall