On Mon, Apr 8, 2013 at 9:13 PM, Laurent Alfonsi <laurent.alfo...@st.com> wrote: > Hello, > > I have identified a big performance regression between 4.6 and 4.7. (I have > enclosed a pathological test). > > After investigation, it is because of the += statement applied on 2 signed > chars. > - It is now type-promoted to "int" when it is written "result += foo()". > (since 4.7) > - it is type promoted to "unsigned char" when it is written "result = > result + foo()". > > The "char->int->char" cast is blocking some optimizations in later phases. > Anyway, this doesn't look wrong, so I extended fold optimization in order to > catch this case. (patch enclosed) > The patch basically transforms : > (TypeA) ( (TypeB) a1 + (TypeB) a2 ) /* with a1 and a2 of > the signed type TypeA */ > into : > a1 + a2 > > I believe this is legal for any licit a1/a2 input values (no overflow on > signed char). > No new failure on the two tested targets : sh-superh-elf and > x86_64-unknown-linux-gnu. > Should I enter a bugzilla to track this ? Is it ok for trunk ?
Please open a bugzilla. No, the patch is not ok, as said by Marc. It's possible to shorten the operation by performing it using unsigned arithmetic, that is (signed)((unsigned)a1 + (unsigned)a2). But if really "casts" cause the issue you are seeing that will not help you. Richard. > 2013-04-08 Laurent Alfonsi <laurent.alfo...@st.com> > > * fold-const.c (fold_unary_loc): Suppress useless type promotion. > > > Thanks, > Laurent >
