On Mon, Oct 26, 2020 at 11:06:22AM +0000, Alex Coplan wrote: > Well, only the low 32 bits of the subreg are valid. But because those > low 32 bits are shifted left 2 times, the low 34 bits of the ashift are > valid: the bottom 2 bits of the ashift are zeros, and the 32 bits above > those are from the inner SImode reg (with the upper 62 bits being > undefined).
Ugh. Yes, I think you are right. One more reason why we should only use *explicit* sign/zero extends, none of this confusing subreg business :-( > > > diff --git a/gcc/combine.c b/gcc/combine.c > > > index c88382efbd3..fe8eff2b464 100644 > > > --- a/gcc/combine.c > > > +++ b/gcc/combine.c > > > @@ -7419,8 +7419,8 @@ expand_compound_operation (rtx x) > > > } > > > > > > /* If we reach here, we want to return a pair of shifts. The inner > > > - shift is a left shift of BITSIZE - POS - LEN bits. The outer > > > - shift is a right shift of BITSIZE - LEN bits. It is arithmetic or > > > + shift is a left shift of MODEWIDTH - POS - LEN bits. The outer > > > + shift is a right shift of MODEWIDTH - LEN bits. It is arithmetic or > > > logical depending on the value of UNSIGNEDP. > > > > > > If this was a ZERO_EXTEND or ZERO_EXTRACT, this pair of shifts will > > > be > > > > MODEWIDTH isn't defined here yet, it is initialised just below to > > MODE_PRECISION (mode). > > Yes, but bitsize isn't defined at all in this function AFAICT. Are > comments not permitted to refer to variables defined immediately beneath > them? Of course you can -- comments are free form text after all -- but as written it suggest there already is an initialised variable "modewidth". Just move the initialisation to above this comment? Segher
