A declaration of a local label is indeed a number followed by a colon. But a reference to a local label needs a suffix of f or b, depending on whether you want to look forwards or backwards – i.e. 1f refers to the next 1: label in the forwards direction. So declaring the label … 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
There is no support for inline assembly, but you can link with code written in assembly through C, compiling with cgo and using import “C”, like in gmp.go. You could alternatively write in a style of assembly that is directly compatible with Go, like in asm_linux_amd64.s, that requires that function names start with “ยท”. Or, … Read more
There’s a massive difference between MSVC inline asm and GNU C inline asm. GCC syntax is designed for optimal output without wasted instructions, for wrapping a single instruction or something. MSVC syntax is designed to be fairly simple, but AFAICT it’s impossible to use without the latency and extra instructions of a round trip through … Read more
This one has no branches, and doesn’t suffer from overflow or underflow: return (a > b) – (a < b); With gcc -O2 -S, this compiles down to the following six instructions: xorl %eax, %eax cmpl %esi, %edi setl %dl setg %al movzbl %dl, %edx subl %edx, %eax Here’s some code to benchmark various compare … Read more
The __volatile__ modifier on an __asm__ block forces the compiler’s optimizer to execute the code as-is. Without it, the optimizer may think it can be either removed outright, or lifted out of a loop and cached. This is useful for the rdtsc instruction like so: __asm__ __volatile__(“rdtsc”: “=a” (a), “=d” (d) ) This takes no … Read more
If you are using separate assembly files, gas has a directive to support Intel syntax: .intel_syntax noprefix # not recommended for inline asm which uses Intel syntax and doesn’t need the % prefix before register names. (You can also run as with -msyntax=intel -mnaked-reg to have that as the default instead of att, in case … Read more
First, you have to understand the concept of rings. A kernel runs in ring 0, meaning it has a full access to memory and opcodes. A program runs usually in ring 3. It has a limited access to memory, and cannot use all the opcodes. So when a software need more privileges (for opening a … Read more
The interactions with optimisations are explained about halfway down the “Assembler Instructions with C Expression Operands” page in the documentation. GCC doesn’t try to understand any of the actual assembly inside the asm; the only thing it knows about the content is what you (optionally) tell it in the output and input operand specification and … Read more
Using GCC __asm__(“movl %edx, %eax\n\t” “addl $2, %eax\n\t”); Using VC++ __asm { mov eax, edx add eax, 2 }