[cfe-dev] MRV on x86_64

[Eli Friedman]

On Thu, Mar 10, 2011 at 1:09 PM, Arushi Aggarwal <arushi987 at gmail.com> wrote:
> Hi,
>
> I am using clang on the following code,
>
> struct S {
>   float a,b,c;
> };
>
> void foo(struct S obj) {
>   obj.a = 1.2;
> }
>
> int main() {
>   struct S s;
>   s.a = 1.0;
>   s.b = 1.0;
>   s.c = 1.0;
>   foo(s);
>   return 0;
> }
>
> The generated code for foo, changes the type of the argument to foo to be
> {double, double} .
>
> llvm-gcc was converting this to {double, float} which was an ABI
> requirement, as par an older
> discussion(http://lists.cs.uiuc.edu/pipermail/llvmdev/2010-January/028870.html)
>
> Why does clang change it to a struct with a different size?

clang uses different code written from scratch, so there are
differences in the output.  In this case, it's a missed optimization;
ideally, I think the signature of foo would be @_Z3foo1S(<2 x float>,
float).

-Eli