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What is the purpose of the nop in this code?
Posted: Thu Nov 13, 2025 3:12 am
by sharpice
Hi everyone, firs post here.
I am exploring the RISC-V ISA and in particular assembly while also catching up on C. I have a setup where I write simple C programs and compile them to assembly.
I compiled the following program:
Code: Select all
void bob() {
static int z = 0;
z += 1;
}
int valio() {
auto int a = 6;
bob();
bob();
return a;
}
and got the following assembly code (snippet):
Code: Select all
...
bob:
addi sp,sp,-16
sw ra,12(sp)
sw s0,8(sp)
addi s0,sp,16
lui a5,%hi(z.0)
lw a5,%lo(z.0)(a5)
addi a4,a5,1
lui a5,%hi(z.0)
sw a4,%lo(z.0)(a5)
nop
lw ra,12(sp)
lw s0,8(sp)
addi sp,sp,16
jr ra
...
What is the purpose of this "nop" here? I understand that it is supposed to do nothing and consume 1 cycle, if I am not mistaken. What I am trying to learn more about it why GCC decided to put it there and what purpose it servers.
I compiled it via:
Code: Select all
riscv64-unknown-elf-gcc -O0 -nostdlib -march=rv32i -mabi=ilp32 -Wl,-Tm.ld c-asm.c -S
What do I need to read more about to understand this?
Re: What is the purpose of the nop in this code?
Posted: Thu Nov 13, 2025 7:10 am
by Demindiro
AFAICT it serves no purpose. clang doesn't generate it and it's gone with -O1 or higher.
It is strange it is emitted at all though. You can reproduce it with just void empty() {}:
Code: Select all
empty:
addi sp,sp,-16
sd ra,8(sp)
sd s0,0(sp)
addi s0,sp,16
nop
ld ra,8(sp)
ld s0,0(sp)
addi sp,sp,16
jr ra
Function prologue, body then epilogue. Must be a detail of the compilation process leaking through.
I guess you'll only find a real answer by looking at GCC source.
Re: What is the purpose of the nop in this code?
Posted: Thu Nov 13, 2025 7:53 am
by Demindiro
Did a bit more research and it turns out the same happens on x86-64:
Code: Select all
empty:
push rbp
mov rbp, rsp
nop
pop rbp
ret
bob:
push rbp
mov rbp, rsp
mov eax, DWORD PTR z.0[rip]
add eax, 1
mov DWORD PTR z.0[rip], eax
nop
pop rbp
ret
I guess I never noticed because I don't bother looking at unoptimized assembly
Re: What is the purpose of the nop in this code?
Posted: Thu Nov 13, 2025 8:50 am
by sharpice
Huh, interesting! I guess I put too much meaning into it. It did disappear with -O1.
One thing I noticed initially when compiling with -O0 is that the resulting assembly, although technically correct, is pretty bad. I am guessing this level of optimization is not used at all? Should I avoid using it even for learning purposes? Want to avoid creating bad habits from what I am reading.
Re: What is the purpose of the nop in this code?
Posted: Thu Nov 13, 2025 9:25 am
by Demindiro
-O0 applies next to no optimizations. Only very trivial ones like substituting mul/div with shifts.
If I read compiled assembly it's usually to figure out if it optimizes well or if there's a miscompilation. -O0 might be useful for debugging, though even then you're probably better of with -Og.
I first learned to read x86-64 assembly by trying to optimizing interpreters. I found that at some point optimizing turned into guesswork and I had to understand what was being generated before I could go any further.
I then learned to write RISC-V assembly by consulting the specification and writing my own OS for it.