[Bug c/91526] Unnecessary SSE and other instructions generated when compiling in C mode (vs. C++ mode)

[rguenth at gcc dot gnu.org]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=91526

Richard Biener <rguenth at gcc dot gnu.org> changed:

           What    |Removed                     |Added
----------------------------------------------------------------------------
           Keywords|                            |missed-optimization
             Target|                            |x86_64-*-*, i?86-*-*
             Status|UNCONFIRMED                 |NEW
   Last reconfirmed|                            |2019-08-23
                 CC|                            |jakub at gcc dot gnu.org,
                   |                            |mpolacek at gcc dot gnu.org
          Component|target                      |c
     Ever confirmed|0                           |1

--- Comment #1 from Richard Biener <rguenth at gcc dot gnu.org> ---
I think I've seen duplicates about this issue where C/C++ differ in the IL
presented to the middle-end for the aggregate return stmt which in the
end causes us to not elide an aggregate copy.  Usually SRA deals with
this but it has a hard job with heuristics and arrays...

The C++ FE does

;; Function Vec multiply(const Vec*, const Vec*) (null)
;; enabled by -tree-original


{
  struct Vec result [value-expr: <retval>];
^^^

while the C FE does

;; Function multiply (null)
;; enabled by -tree-original


{
  struct Vec result;

so the C++ FE already elides the return copy by placing 'result' in the
return slot while the C FE doesn't do this.

Let's make this a C enhancement request rather than a missed optimization
during GIMPLE optimizations (which there are dups for already).

Marek - any chance the C FE could do sth like this?  Maybe we can also
do this during gimplification, we'd have to see what constraints the C++
FE has for performing this.