llvm dev - Apr 2018 - const struct auto vs. static

I find it surprising that given this:
void f1()
{
const struct { int a; } b = { 0 };
}


that clang does not put b on the stack.
gcc does. My gut feeling says it belongs on stack.
But perhaps, the thing is, it is const, and location
shouldn't matter to most code, as long as lifetime is adequate.
Almost no code could tell the difference.


But we have code that takes the address of things, and other things
and wants them in range, i.e. all on the stack.

I had to remove const to make it work.

I realize such code is troublesome in the face of inlining.
We should probably mark some of our functions to not be inlined.

$ cat 3.c
void f2(const void*);

void f1()
{
const struct { int a; } b = { 0 };
f2(&b);
}
$ clang -c -S 3.c
$ cat 3.s
_f1:                                    ## @f1
	pushq	%rbp
	movq	%rsp, %rbp
	leaq	_f1.b(%rip), %rax
	movq	%rax, %rdi
	callq	_f2
	popq	%rbp
	retq


$ clang --version
Apple LLVM version 9.0.0 (clang-900.0.39.2)
Target: x86_64-apple-darwin16.7.0
Thread model: posix
InstalledDir:
/Applications/Xcode.app/Contents/Developer/Toolchains/XcodeDefault.xctoolchain/usr/bin


Thank you,
 - Jay

On 4/1/2018 10:31 PM, Jay K via llvm-dev wrote:
> I find it surprising that given this:
> void f1()
> {
> const struct { int a; } b = { 0 };
> }
>
>
> that clang does not put b on the stack.
> gcc does. My gut feeling says it belongs on stack.
> But perhaps, the thing is, it is const, and location
> shouldn't matter to most code, as long as lifetime is adequate.
> Almost no code could tell the difference.
This is controlled by -fmerge-all-constants/-fno-merge-all-constants.  
It's on by default with clang, off by default with gcc.

-Eli

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