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

[joseph at codesourcery dot com]

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

--- Comment #7 from joseph at codesourcery dot com <joseph at codesourcery dot com> ---
There's more or less the same ABI question as in bug 91398 about whether 
there is any constraint on the called function writing to the return value 
slot in cases where it does not return normally.

Supposing the ABI allows the return value slot (register or memory) to be 
written to by the called function even if it does not end up returning 
normally, then the optimization in this bug would be valid, while that in 
bug 91398 would not be valid if non-normal return is a possibility.  (The 
example in the present bug also doesn't allow non-normal return, unless we 
say longjmp from a SIGFPE handler is OK - is -fnon-call-exceptions only 
needed for language exceptions or also for longjmp?)

(Validity would also depend on it not affecting the observed address of 
the variable "result" in such a way as to make it equal to the observed 
address of some object in a calling function - but I expect the 
interesting cases for this optimization are where the variable is only 
stored to, not ones where addresses get compared, if it's even possible 
for the same return value slot to get used in more than one function on 
the call stack.)