Why does gcc create redundant assembly code?
assembly, c, compiler-construction, gcc, optimization
Solution
I thought gcc should be able to optimize away.
From the gcc manual:
Without any optimization option, the compiler's goal is to reduce the cost of compilation and to make debugging produce the expected results.
In other words, it doesn't optimize unless you ask it to. When I turn on optimizations using the `-O3` flag, gcc 4.4.6 produces much more efficient code:
bar:
.LFB0:
.cfi_startproc
leaq 4(%rdi), %rax
ret
.cfi_endproc
For more details, see Options That Control Optimization in the manual.
Problem
I wanted to look into how certain C/C++ features were translated into assembly and I created the following file: ``` struct foo { int x; char y[0]; }; char *bar(struct foo *f) { return f->y; } ``` I then compiled this with `gcc -S` (and also tried with `g++ -S`) but when I looked at the assembly code, I was disappointed to find a trivial redundancy in the bar function that I thought `gcc` should be able to optimize away: ``` _bar: Leh_func_begin1: pushq %rbp Ltmp0: movq %rsp, %rbp Ltmp1: movq %rdi, -8(%rbp) movq -8(%rbp), %rax movabsq $4, %rcx addq %rcx, %rax movq %rax, -24(%rbp) movq -24(%rbp), %rax movq %rax, -16(%rbp) movq -16(%rbp), %rax popq %rbp ret Leh_func_end1: ``` Among other things, the lines ``` movq %rax, -24(%rbp) movq -24(%rbp), %rax movq %rax, -16(%rbp) movq -16(%rbp), %rax ``` seem pointlessly redundant. Is there any reason gcc (and possibly other compilers) cannot/does not optimize this away?