Testing whether use of local variables changes optimized machine code

ex1.c

#import <stdio.h>

int __attribute__((noinline)) foo(void) {
	return 1;
}
int __attribute__((noinline)) bar(void) {
	return 2;
}
int __attribute__((noinline)) baz(int a1, int a2) {
	return a1 + a2;
}

int main() {
	printf("%d\n", baz(foo(), bar()));
	return 0;
}

ex2.c

#import <stdio.h>

int __attribute__((noinline)) foo(void) {
	return 1;
}
int __attribute__((noinline)) bar(void) {
	return 2;
}
int __attribute__((noinline)) baz(int a1, int a2) {
	return a1 + a2;
}

int main() {
	int qux = foo();
	int quux = bar();
	int corge = baz(qux, quux);
	printf("%d\n", corge);
	return 0;
}

make.sh

clang -S -emit-llvm ex1.c
clang -S -emit-llvm ex2.c
clang -O -S -mllvm --x86-asm-syntax=intel ex1.c
clang -O -S -mllvm --x86-asm-syntax=intel ex2.c

Output

$ uname -a
Linux massachusetts 4.4.0-18-generic #34-Ubuntu SMP Wed Apr 6 14:01:02 UTC 2016 x86_64 x86_64 x86_64 GNU/Linux


$ clang -v
clang version 3.8.0-2ubuntu1 (tags/RELEASE_380/final)
Target: x86_64-pc-linux-gnu
Thread model: posix
InstalledDir: /usr/bin
Found candidate GCC installation: /usr/bin/../lib/gcc/i686-linux-gnu/5.3.1
Found candidate GCC installation: /usr/bin/../lib/gcc/i686-linux-gnu/6.0.0
Found candidate GCC installation: /usr/bin/../lib/gcc/x86_64-linux-gnu/5.3.1
Found candidate GCC installation: /usr/bin/../lib/gcc/x86_64-linux-gnu/6.0.0
Found candidate GCC installation: /usr/lib/gcc/i686-linux-gnu/5.3.1
Found candidate GCC installation: /usr/lib/gcc/i686-linux-gnu/6.0.0
Found candidate GCC installation: /usr/lib/gcc/x86_64-linux-gnu/5.3.1
Found candidate GCC installation: /usr/lib/gcc/x86_64-linux-gnu/6.0.0
Selected GCC installation: /usr/bin/../lib/gcc/x86_64-linux-gnu/5.3.1
Candidate multilib: .;@m64
Selected multilib: .;@m64


$ diff ex1.ll ex2.ll
1c1
< ; ModuleID = 'ex1.c'
---
> ; ModuleID = 'ex2.c'
31a32,34
>   %qux = alloca i32, align 4
>   %quux = alloca i32, align 4
>   %corge = alloca i32, align 4
33a37
>   store i32 %2, i32* %qux, align 4
35,36c39,45
<   %4 = call i32 @baz(i32 %2, i32 %3)
<   %5 = call i32 (i8*, ...) @printf(i8* getelementptr inbounds ([4 x i8], [4 x i8]* @.str, i32 0, i32 0), i32 %4)
---
>   store i32 %3, i32* %quux, align 4
>   %4 = load i32, i32* %qux, align 4
>   %5 = load i32, i32* %quux, align 4
>   %6 = call i32 @baz(i32 %4, i32 %5)
>   store i32 %6, i32* %corge, align 4
>   %7 = load i32, i32* %corge, align 4
>   %8 = call i32 (i8*, ...) @printf(i8* getelementptr inbounds ([4 x i8], [4 x i8]* @.str, i32 0, i32 0), i32 %7)


$ diff ex1.s ex2.s
3c3
< 	.file	"ex1.c"
---
> 	.file	"ex2.c"