[PATCH] rs6000: -flto forgets 'no-vsx' function attributes (PR target/70010)

[Richard Biener]

On October 14, 2019 5:31:58 PM GMT+02:00, Peter Bergner <bergner@linux.ibm.com> wrote:
>On 10/12/19 3:46 AM, Segher Boessenkool wrote:
>> Two spaces after a period.  How about something like
>> 
>>       /* Callee's options should be a subset of the caller's.  Also,
>a function
>> 	 without VSX enabled should not be inlined into one with VSX
>enabled,
>> 	 because it may be important it is disabled there; see PR70010.  */
>> 
>> It's not clear to me why this is important, and what makes -mvsx
>different
>> from all other similar options?
>
>I agree, there is nothing special about VSX here and the other similar
>options
>like Altivec, HTM, etc., etc. should all be handled similarly.
>
>I agree with your other comment that we should be looking at explicit
>option
>usage versus default options.  However, the way we now implement
>default CPU,
>the gcc driver always passes a -mcpu= option to cc1 no matter if the
>user
>used -mcpu= or not, so -mcpu= will always looks like an explicit
>option.
>So when -mcpu=power[789] is passed to cc1 (via explicit user usage or
>default
>cpu), does that look like -mvsx was also explicitly used?  I'm guessing
>not.
>
>So if we have a caller compiled with -mcpu=power8 (VSX and Altivec are
>implicitly
>enabled) and a callee compiled with -mcpu=power6 (VSX and Altivec is
>not enabled
>...implicitly), do we allow inlining?  I would say we shouldn't, but
>the VSX
>and Altivec flags in the callee are a subset of the caller's flags.  It
>must
>be that the ISA* flags in rs6000_isa_flags that save us from not
>inlining?
>
>Therefore, I'd say that the callee's flags should be a subset of the
>caller's
>flags as the current code does now, but we should be also checking that
>the
>callee doesn't have an explicitly used option flag(s) that conflicts
>with
>the callers flags (implicit or explicit).  That means the caller's
>flags
>must match exactly the callee's flags, for those flags that were
>explicitly
>set in the callee.

The general case should be that if the caller ISA supports the callee one then inlining is OK. If this is not wanted in some cases then there are options like using a noinline attribute. 

>
>Peter