Questions tagged [inline-assembly]

Assembly that is embedded within a source in another, higher language, such as x86 assembly embedded in C or C++.

Inline assembly is used in higher level language to provide access to features not exposed by intrinsics. c and c++ are the most common "host" languages that allow inline asm.

Don't use inline asm without being aware of the potential performance downsides, as well as the obvious maintainability / portability downsides. The compiler can't understand inline asm for constant-propagation and other optimizations. If you can get the compiler to generate equivalent asm from normal source code without inline asm, that is almost always preferable.

Resources

https://gcc.gnu.org/wiki/DontUseInlineAsm. Step zero in using inline asm is to try to avoid needing it in the first place.
Visual C++ inline assembly
GCC inline assembly HOWTO, from 2003 but still correct and useful.
GNU C inline asm official docs
Another more recent GNU C tutorial
A nice step-by-step explanation for beginners of using the right constraints for input and output operands for (idiv). The other answer on that question also explains when and why to use early-clobber & in output operands.
Basics of using "r" and "=r" inputs/outputs, or a "+r" in/out operand. Some background on how GNU C inline asm syntax requires the operands to describe the template to the compiler.
Why can't local variable be used in GNU C basic inline asm statements? - local vars
Using dummy "m" or "=m" operands to tell the compiler which memory you read/write via pointers you asked for in "r" operands, instead of using a "memory" clobber:
- How can I indicate that the memory *pointed* to by an inline ASM argument may be used?
- a simple strlen (telling the compiler a whole block of memory is referenced without an explicit length, using a dummy struct with flexible array member).
- Looping over arrays with inline assembly (merits of "m" vs. "r" constraints)
GNU C inline asm link collection at the bottom of this answer (guides / advice / info)
MSVC inline asm vs. GNU C inline asm for wrapping a single instruction
Referencing memory operands in .intel_syntax GNU C inline assembly - don't use .intel_syntax inside an asm statement; use compiler options or dialect alternatives.

The x86 tag wiki has tons of good stuff for that architecture, and the assembly tag wiki also has a few links.

Making a function-call from inline asm: avoid if possible

Direct C function call using GCC's inline assembly - avoid calling from inline asm if you can (because it's hard to make it safe), but %P0 can expand to a function name.
Calling printf in extended inline ASM - hopefully-complete clobber list of call-clobbered registers for x86-64, and safely handling the red-zone, if you really insist on making a call from inline asm.

Non-x86

GNU C inline asm works the same way for non-x86 architectures (you still use input and output operands with constraints to get data into / out of asm statements).

There are differences: many targets don't have a constraint syntax for requesting a specific register (e.g. x86's "a" for eax/rax).

2169 questions

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Can register name be passed into assembly template in GCC inline assembly

unsigned int read_reg(char *reg_name) { unsigned int result; __asm__ __volatile__ ("sw xxx, %0" : : "r"(result)); return result; } The above is a piece of RISC-V code. reg_name is got from CLI, and it's a string, such as "ra" or "x1",…

gcc inline-assembly riscv

asked Oct 21 '21 at 10:42

Wanghz

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1 answer

inline ARMv7-M assembly code into c using gcc (Load Register literal)

I would like to be able to have LDR literal instructions to load registers. I have: extern uint32_t _vStackBase; extern uint32_t _vStackTop; __asm volatile ( " mov r10, #0x5a5a5a5a \n" " ldr r8,…

gcc arm inline-assembly armv7

asked Oct 13 '21 at 10:29

mastupristi

1,240
1
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29

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1 answer

Is there a non inline asm version of AngelScript?

I recently worked a bit with AngelScript and came across some linker errors when trying to compile in x64. If I try to compile my c++ code in x86 everything works fine. After a bit of research I noticed that inline assembly is not supported anymore…

c++ windows x86-64 inline-assembly angelscript

asked Oct 08 '21 at 23:26

user13455484

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0 answers

Program does not segfault in gdb

I have this attempt to switch to a different stack by setting %rbp and %rsp: #include #include void* main_rbp = NULL; void* main_rsp = NULL; int run_stack() { int one = 1; printf("Hello from my new stack!\n"); //…

c gdb inline-assembly

asked Sep 18 '21 at 15:10

ssmid

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0 answers

ARM Inline assembly for C code not executing

I am trying to use inline assembly for an ARM C code. My 'main.c' code looks something like this: ... void jump() { __asm { B 0x15000 } } INT32 main(void) { ... Write val1 into register 1 jump(); Write val2 into register 2 …

c arm inline-assembly armcc

asked Sep 03 '21 at 16:33

TJ1

7,578
19
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1 answer

how to insert inline assembly instruction by using llvm pass

I tried to insert an assembly instruction into each base block using pass in the IR Pass of LLVM. Update: LLVMContext *Ctx = nullptr; Ctx = &M.getContext(); BasicBlock::iterator IP = BB.getFirstInsertionPt(); IRBuilder<> IRB(&(*IP)); StringRef…

llvm inline-assembly

asked Sep 03 '21 at 13:31

gazile

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0 answers

far return gcc cross-compiler

I have a C function in my custom kernel that prints DEBUG on the top left corner. I call that function using a different cs : asm("lcall $0x28, $0x0"); (The code is copied at the beggining of the 0x28 code segment). In the c function that prints…

c gcc x86 inline-assembly memory-segmentation

asked Aug 31 '21 at 13:05

Leosa99 _

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0 answers

how to "nop ptr[eax + 0x00]" on C++ _asm inline

i have this code in asm: cmp ecx,46DC0E00 je 1A380021 nop dword ptr [eax+00] mov [edx+00000584],ecx mov [edx+00000584],00000000 jmp game.exe+A42067 mov [edx+00000584],ecx jmp game.exe+A42067 and i have this code on C++: __asm { cmp ecx,…

c++ assembly visual-c++ inline-assembly no-op

asked Aug 12 '21 at 04:18

devNotonePiece

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0 answers

"asm volatile ("" ::: "memory");" does not work in c++

I was trying to follow the queue implementation in this blog that uses a memory barries like below: asm volatile ("" ::: "memory"); But the same is not working for me and throwing this error: error C2589: ':': illegal token on right side of…

assembly visual-c++ inline-assembly memory-barriers

asked Aug 07 '21 at 16:30

Sayantan Ghosh

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2 answers

assembly - Issue with permutations generation

I'm trying to list all the permutations of the first N natural numbers (N <= 6) with MSVC Assembly 8086 inline. I can't change anything of the C program, I can only manage the assembly code. So this is what I've done (and works with N <= 3), using…

c assembly x86 permutation inline-assembly

asked Jul 24 '21 at 14:35

maicol07

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3 answers

Why doesn't this MSVC asm block have a ret, or the non-void function have a return?

I'm learning about using inline assembly inside the C++ code. Here is the very simple example: // Power2_inline_asm.c // compile with: /EHsc // processor: x86 #include int power2( int num, int power ); int main( void ) { printf_s(…

c++ assembly visual-c++ inline-assembly

asked Jul 19 '21 at 09:28

derek911

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0 answers

What is the most efficient way to pass a local variable address to extended inline ASM for ARM-v7?

Consider this code snippet: __asm volatile ( " MOVW R0, #0x0000 \n\t" " MOVT R0, #0x3004 \n\t" " LDRSH R1, [R0, #12] \n\t" ⋮ ) This is the hard-coded way for loading what's stored…

embedded inline-assembly local-variables armv7

asked Jul 17 '21 at 16:04

Hansel

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1 answer

Homemade kernel Inline assembly gives bad register name error. C

I am coding a custom kernel (some of the work comes from https://www.codeproject.com/Articles/664165/Writing-a-boot-loader-in-Assembly-and-C-Part). Here is the error test.c: Assembler messages: test.c:12: Error: bad register name `%dil' ld: cannot…

c assembly kernel inline-assembly cpu-registers

asked Jul 16 '21 at 14:24

TechTheGuy

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1 answer

inline assembly function pointer calling

minimalist example: void someFunction(){ std::cout << "Hello World" << std::endl; } void (*functionPointer)(); functionPointer = someFunction; int main(){ __asm__("call *%P0"::"m"(functionPointer):); return 0; } In gcc 10.3.0, this results…

pointers x86-64 inline-assembly

asked Jul 13 '21 at 14:47

mingcr

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1 answer

How can use asm() with a const char* instead of a string literal?

If I'd do that: const char *str = "some assembly instructions"; asm(str); CLion'd say "Expected string literal in 'asm'"

c assembly inline-assembly string-literals

asked Jul 10 '21 at 07:50

Sivyziom

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