C++ – How to remove “noise” from GCC/clang assembly output

Stripping out the .cfi directives, unused labels, and comment lines is a solved problem: the scripts behind Matt Godbolt's compiler explorer are open source on its github project. It can even do colour highlighting to match source lines to asm lines (using the debug info).

You can set it up locally so you can feed it files that are part of your project with all the #include paths and so on (using -I/...). And so you can use it on private source code that you don't want to send out over the Internet.

Matt Godbolt's CppCon2017 talk “What Has My Compiler Done for Me Lately? Unbolting the Compiler's Lid” shows how to use it (it's pretty self-explanatory but has some neat features if you read the docs on github), and also how to read x86 asm, with a gentle introduction to x86 asm itself for total beginners, and to looking at compiler output. He goes on to show some neat compiler optimizations (e.g. for dividing by a constant), and what kind of functions give useful asm output for looking at optimized compiler output (function args, not int a = 123;).

With plain gcc/clang (not g++), -fno-asynchronous-unwind-tables avoids .cfi directives. Possibly also useful: -fno-exceptions -fno-rtti -masm=intel. Make sure to omit -g.

Copy/paste this for local use:

g++ -fno-asynchronous-unwind-tables -fno-exceptions -fno-rtti -fverbose-asm \
    -Wall -Wextra  foo.cpp   -O3 -masm=intel -S -o- | less

But really, I'd recommend just using Godbolt directly (online or set it up locally)! You can quickly flip between versions of gcc and clang to see if old or new compilers do something dumb. (Or what ICC does, or even what MSVC does.) There's even ARM / ARM64 gcc 6.3, and various gcc for PowerPC, MIPS, AVR, MSP430. (It can be interesting to see what happens on a machine where int is wider than a register, or isn't 32-bit. Or on a RISC vs. x86).

For C instead of C++, use -xc -std=gnu11 or something; the compiler explorer site only provides g++ / clang++, not gcc / clang. (Or you can use C mode in the language dropdown, but that has a different selection of compilers that's mostly more limited. And it resets your source pane so it's more of an ordeal to flip between C and C++.)

Useful compiler options for making asm for human consumption:

• Remember, your code only has to compile, not link: passing a pointer to an external function like void ext(void*p) is a good way to stop something from optimizing away. You only need a prototype for it, with no definition so the compiler can't inline it or make any assumptions about what it does. (Or inline asm like Benchmark::DoNotOptimize can force a compiler to materialize a value in a register, or forget about it being a known constant, if you know GNU C inline asm syntax well enough to use constraints to understand the effect you're having on what you're requiring of the compiler.)

• I'd recommend using -O3 -Wall -Wextra -fverbose-asm -march=haswell) for looking at code. (-fverbose-asm can just make the source look noisy, though, when all you get are numbered temporaries as names for the operands.) When you're fiddling with the source to see how it changes the asm, you definitely want compiler warnings enabled. You don't want to waste time scratching your head over the asm when the explanation is that you did something that deserves a warning in the source.

• To see how the calling convention works, you often want to look at caller and callee without inlining.

  You can use __attribute__((noipa)) foo_t foo(bar_t x) { ... } on a definition, or compile with gcc -O3 -fno-inline-functions -fno-inline-functions-called-once -fno-inline-small-functions to disable inlining. (But those command line options don't disable cloning a function for constant-propagation. noipa = no Inter-Procedural Analysis. It's even stronger than __attribute__((noinline,noclone)).) See From compiler perspective, how is reference for array dealt with, and, why passing by value(not decay) is not allowed? for an example.

  Or if you just want to see how functions pass / receive args of different types, you could use different names but the same prototype so the compiler doesn't have a definition to inline. This works with any compiler. Without a definition, a function is just a black box to the optimizer, governed only by the calling convention / ABI.

• -ffast-math will get many libm functions to inline, some to a single instruction (esp. with SSE4 available for roundsd). Some will inline with just -fno-math-errno, or other "safer" parts of -ffast-math, without the parts that allow the compiler to round differently. If you have FP code, definitely look at it with/without -ffast-math. If you can't safely enable any of -ffast-math in your regular build, maybe you'll get an idea for a safe change you can make in the source to allow the same optimization without -ffast-math.

• -O3 -fno-tree-vectorize will optimize without auto-vectorizing, so you can get full optimization without if you want to compare with -O2 (which doesn't enable autovectorization on gcc, but does on clang).

• clang unrolls loops by default, so -fno-unroll-loops can be useful in complex functions. You can get a sense of "what the compiler did" without having to wade through the unrolled loops. (gcc enables -funroll-loops with -fprofile-use, but not with -O3). (This is a suggestion for human-readable code, not for code that would run faster.)

• Definitely enable some level of optimization, unless you specifically want to know what -O0 did. Its "predictable debug behaviour" requirement makes the compiler store/reload everything between every C statement, so you can modify C variables with a debugger and even "jump" to a different source line within the same function, and have execution continue as if you did that in the C source. -O0 output is so noisy with stores/reloads (and so slow) not just from lack of optimization, but forced de-optimization to support debugging. (also related).

To get a mix of source and asm, use gcc -Wa,-adhln -c -g foo.c | less to pass extra options to as. (More discussion of this in a blog post, and another blog.). Note that the output of this isn't valid assembler input, because the C source is there directly, not as an assembler comment. So don't call it a .s. A .lst might make sense if you want to save it to a file.

Godbolt's color highlighting serves a similar purpose, and is great at helping you see when multiple non-contiguous asm instructions come from the same source line. I haven't used that gcc listing command at all, so IDK how well it does, and how easy it is for the eye to see, in that case.

I like the high code density of godbolt's asm pane, so I don't think I'd like having source lines mixed in. At least not for simple functions. Maybe with a function that was too complex to get a handle on the overall structure of what the asm does...

And remember, when you want to just look at the asm, leave out the main() and the compile-time constants. You want to see the code for dealing with a function arg in a register, not for the code after constant-propagation turns it into return 42, or at least optimizes away some stuff.

Removing static and/or inline from functions will produce a stand-alone definition for them, as well as a definition for any callers, so you can just look at that.

Don't put your code in a function called main(). gcc knows that main is special and assumes it will only be called once, so it marks it as "cold" and optimizes it less.

The other thing you can do: If you did make a main(), you can run it and use a debugger. stepi (si) steps by instruction. See the bottom of the x86 tag wiki for instructions. But remember that code might optimize away after inlining into main with compile-time-constant args.

__attribute__((noinline)) may help, on a function that you want to not be inlined. gcc will also make constant-propagation clones of functions, i.e. a special version with one of the args as a constant, for call-sites that know they're passing a constant. The symbol name will be .clone.foo.constprop_1234 or something in the asm output. You can use __attribute__((noclone)) to disable that, too.).

For example

If you want to see how the compiler multiplies two integers: I put the following code on the Godbolt compiler explorer to get the asm (from gcc -O3 -march=haswell -fverbose-asm) for the wrong way and the right way to test this.

// the wrong way, which people often write when they're used to creating a runnable test-case with a main() and a printf
// or worse, people will actually look at the asm for such a main()
int constants() { int a = 10, b = 20; return a * b; }
    mov     eax, 200  #,
    ret                     # compiles the same as  return 200;  not interesting

// the right way: compiler doesn't know anything about the inputs
// so we get asm like what would happen when this inlines into a bigger function.
int variables(int a, int b) { return a * b; }
    mov     eax, edi  # D.2345, a
    imul    eax, esi        # D.2345, b
    ret

(This mix of asm and C was hand-crafted by copy-pasting the asm output from godbolt into the right place. I find it's a good way to show how a short function compiles in SO answers / compiler bug reports / emails.)