Re: [PATCH/RFC] rs6000: Remove optimize_for_speed check for implicit TARGET_SAVE_TOC_INDIRECT [PR108184]

[Michael Meissner <meissner@linux.ibm.com>]

On Mon, Jan 16, 2023 at 05:39:04PM +0800, Kewen.Lin wrote:
> Hi,
> 
> Now we will check optimize_function_for_speed_p (cfun) for
> TARGET_SAVE_TOC_INDIRECT if it's implicitly enabled.  But
> the effect of -msave-toc-indirect is actually to save the
> TOC in the prologue for indirect calls rather than inline,
> it's also good for optimize_function_for_size?  So this
> patch is to remove the check of optimize_function_for_speed
> and make it work for both optimizing for size and speed.
> 
> Bootstrapped and regtested on powerpc64-linux-gnu P8,
> powerpc64le-linux-gnu P{9,10} and powerpc-ibm-aix.
> 
> Any thoughts?
> 
> Thanks in advance!

Well in terms of size, it is only a savings if we have 2 or more indirect calls
within a module, and we are not compiling for power10.

On power9, if we have just one indirect call, then it is the same size.

On power10, the -msave-toc-indirect switch does nothing, because we don't need
TOCs when we have prefixed addressing.

So I have objection to the change.  I suspect it may be better with a check for
just optimize either for speed or size, and not for speed.

The option however, can slow things down if there is an early exit to the
function since the store would always be done, even if the function exits
early.

-- 
Michael Meissner, IBM
PO Box 98, Ayer, Massachusetts, USA, 01432
email: meissner@linux.ibm.com