On Fri, Jun 24, 2011 at 3:46 PM, Stubbs, Andrew
<[email protected]> wrote:
> On 24/06/11 09:28, Richard Guenther wrote:
>>> > To be clear, it only skips past NOP_EXPR. Is it not the case that what
>>> > you're describing would need a CONVERT_EXPR?
>> NOP_EXPR is the same as CONVERT_EXPR.
>
> Are you sure?
Yes, definitely. They are synonyms of each other (an unfinished merging
process), the usual check for them is via CONVERT_EXPR_P.
> I thought this was safe because the internals manual says:
>
> NOP_EXPR
> These nodes are used to represent conversions that do not require any
> code-generation ....
>
> CONVERT_EXPR
> These nodes are similar to NOP_EXPRs, but are used in those
> situations where code may need to be generated ....
Which is wrong (sorry).
> So, I tried this example:
>
> int
> foo (int a, short b, short c)
> {
> int bc = b * c;
> return a + (short)bc;
> }
>
> Both before and after my patch, GCC gives:
>
> mul r2, r1, r2
> sxtah r0, r0, r2
>
> (where, SXTAH means sign-extend the third operand from HImode to SImode
> and add to the second operand.)
>
> The dump after the widening_mult pass is:
>
> foo (int a, short int b, short int c)
> {
> int bc;
> int D.2018;
> short int D.2017;
> int D.2016;
> int D.2015;
> int D.2014;
>
> <bb 2>:
> D.2014_2 = (int) b_1(D);
> D.2015_4 = (int) c_3(D);
> bc_5 = b_1(D) w* c_3(D);
> D.2017_6 = (short int) bc_5;
> D.2018_7 = (int) D.2017_6;
> D.2016_9 = D.2018_7 + a_8(D);
> return D.2016_9;
>
> }
>
> Where you can clearly see that the addition has not been recognised as a
> multiply-and-accumulate.
>
> When I step through convert_plusminus_to_widen, I can see that the
> reason it has not matched is because "D.2017_6 = (short int) bc_5" is
> encoded with a CONVERT_EXPR, just as the manual said it would be.
A NOP_EXPR in this place would be valid as well. The merging hasn't
been completed and at least the C frontend still generates CONVERT_EXPRs
in some cases.
> So, according to the manual, and my (admittedly limited) experiments,
> skipping over NOP_EXPR does appear to be safe.
>
> But you say that it isn't safe. So now I'm confused. :(
>
> I can certainly add checks to make sure that the skipped operations
> actually don't make any important changes to the value, but do I need to?
Yes.
Thanks,
Richard.
> Andrew
>
