See the “Extended Asm” page in the GCC documentation. You can prevent an asm instruction from being deleted by writing the keyword volatile after the asm. […] The volatile keyword indicates that the instruction has important side-effects. GCC will not delete a volatile asm if it is reachable. and An asm instruction without any output … Read more
Compile with gcc -masm=intel and don’t try to switch modes inside the asm template string. AFAIK there’s no equivalent before clang14 (Note: MacOS installs clang as gcc / g++ by default.) Also, of course you need to use valid GNU C inline asm, using operands to tell the compiler which C objects you want to … Read more
You can use local variables in extended assembly, but you need to tell the extended assembly construct about them. Consider: #include <stdio.h> int main (void) { int data1 = 10; int data2 = 20; int result; __asm__( ” movl %[mydata1], %[myresult]\n” ” imull %[mydata2], %[myresult]\n” : [myresult] “=&r” (result) : [mydata1] “r” (data1), [mydata2] “r” … Read more
I got the answer from GCC’s mailing list: asm(“call %P0” : : “i”(callee)); // FIXME: missing clobbers Now I just need to find out what %P0 actually means because it seems to be an undocumented feature… Edit: After looking at the GCC source code, it’s not exactly clear what the code P in front of … Read more
From the file gcc/config/i386/i386.c of the GCC sources: b — print the QImode name of the register for the indicated operand. %b0 would print %al if operands[0] is reg 0. w — likewise, print the HImode name of the register. k — likewise, print the SImode name of the register. q — likewise, print the … Read more
In GNU C inline asm, with x86 AT&T syntax: (But https://gcc.gnu.org/wiki/DontUseInlineAsm if you can avoid it). // this example doesn’t really need volatile: the result is the same every time asm volatile(“movl $0, %[some]” : [some] “=r” (x) ); after this, x contains 0. Note that you should generally avoid mov as the first or … Read more
I’m fairly certain you’re being blocked by DEP. The x_CPUIDy_INSNS byte arrays are in a segment of memory marked as data and non-executable. EDIT: That being said, I’ve gotten a version that compiles and runs, but I don’t think gets the right values. Perhaps this will get you along your way. EDIT 2: I think … Read more
implementing alloca actually requires compiler assistance. A few people here are saying it’s as easy as: sub esp, <size> which is unfortunately only half of the picture. Yes that would “allocate space on the stack” but there are a couple of gotchas. if the compiler had emitted code which references other variables relative to esp … Read more
From your original question I did not realize gcc limited red-zone use to leaf functions. I don’t think that’s required by the x86_64 ABI, but it is a reasonable simplifying assumption for a compiler. In that case you only need to make the function calling your assembly routine a non-leaf for purposes of compilation: int … Read more
Editor’s note: this way of using a local register-asm variable is now documented by GCC as “not supported”. It still usually happens to work on GCC, but breaks with clang. (This wording in the documentation was added after this answer was posted, I think.) The global fixed-register variable version has a large performance cost for … Read more