Your quote is from the “calling convention” part of the documentation. At the very least, you do not have to worry about this if you do not call other functions from your assembly code. If you do, then you must respect, among other things, “red zone” and stack alignment considerations, that the recommendation you quote … Read more
Why would it be? The processor core still needs to read the value stored at the memory location, calculate the increment of it, and then store it back. There’s a latency between reading and storing, and in the mean time another operation could have affected that memory location. Even with out-of-order execution, processor cores are … Read more
It’s just call. Use Intel-syntax disassembly if you want to be able to look up instructions in the Intel/AMD manuals. (objdump -drwC -Mintel, GBD set disassembly-flavor intel, GCC -masm=intel) The q operand-size suffix does technically apply (it pushes a 64-bit return address and treats RIP as a 64-bit register), but there’s no way to override … Read more
Alignment is a restriction on which memory positions a value’s first byte can be stored. (It is needed to improve performance on processors and to permit use of certain instructions that works only on data with particular alignment, for example SSE need to be aligned to 16 bytes, while AVX to 32 bytes.) Alignment of … Read more
No, they’re not equivalent. For instance if rax = 1 and rcx = 3, then sub rax, rcx will set the carry flag, because you are subtracting a larger number from a smaller one. But in your second sequence of instructions, following add rax, temp, rax will contain -3 (i.e. 0xfffffffffffffffd), and adding 1 to … Read more
The x86-64 System V ABI has a 128-byte red zone below RSP that is safe from asynchronous clobbering, i.e. is “owned” by the function. It looks like you compiled int foo{ int x = 4; return 0; } with gcc -O0 (optimizations disabled), and gcc chose to keep x in the red-zone instead of adjusting … Read more
Yes, SIMD is possible, as mentioned in comments. You can take advantage of it to parse the HH, MM, and SS parts of the string at the same time. Since you have a 100% fixed format with leading 0s where necessary, this is easier than How to implement atoi using SIMD? – Place-values are fixed … Read more
If you remember C memcpy‘s arg order, and how rep movsb works, that’s most of the way to remembering x86-64 System V. The design makes memcpy(dst, src, size) cheap to implement with rep movsb, except leaving RCX unused in more functions because it’s needed for variable-count shifts more often than anything needs RDX. Then R8 … Read more
In my experience, there is no standard way how C returns a structure. To be able to pass a struct, the compiler usually (invisibly to the user) passes a pointer to the struct, to which the function can copy the contents. How this pointer is passed (first or last on stack) is implementation dependent. Some … Read more
why can’t I use multiple higher bytes in a register Every permutation of an instruction needs to be encoded in the instruction. The original 8086 processor supports the following options: instruction encoding remarks ——————————————————— mov ax,value b8 01 00 <– whole register mov al,value b4 01 <– lower byte mov ah,value b0 01 <– upper … Read more