This is the mail archive of the mailing list for the GCC project.

Index Nav: [Date Index] [Subject Index] [Author Index] [Thread Index]
Message Nav: [Date Prev] [Date Next] [Thread Prev] [Thread Next]
Other format: [Raw text]

Re: PR63633: May middle-end come up width hard regs for insn expanders?

On 10/24/14 08:03, Georg-Johann Lay wrote:
Investigating PR63633 turned out that the middle-end calls insn
expanders with hard registers, namely mulsi3 from avr back-end:

(define_expand "mulsi3"
   [(parallel [(set (match_operand:SI 0 "register_operand" "")
                    (mult:SI (match_operand:SI 1 "register_operand" "")
                             (match_operand:SI 2 "nonmemory_operand" "")))
               (clobber (reg:HI 26))
               (clobber (reg:DI 18))])]

is being called with operands[0] = (reg:SI 22).  This overlaps hard
(reg:DI 18) which extends from R18...R25.

mulsi3 assumes all operands are pseudo registers and consequently the
generated insn raises an ICE in the remainder, and there are other cases
for other expanders where wrong code gets generated because the clobbers
clobber an hard-reg input operand.

Is it in order to have hard registers as arguments to expanders (and
maybe also to insns) like that at expand time (pass .expand) ?

It is easy enough to kick these hard regs into new pseudos in the
respective expanders, however the question arises where the culprit is:

back-end or middle-end?

Until PR63633 I've never seen middle-end coming up with hard regs that
way, thus bunch of avr insns have been written under that -- maybe wrong
-- assumption...
I don't think you can assume you're always going to get a pseudo, though the vast majority of the time you will. Normally it won't matter unless (as noted above) the expander/pattern has references (particularly sets/clobbers) of overlapping hard registers.

It's also probably worth investigating why you got the hard register in the first place as that may indicate something that's wrong or suboptimal either in the expanders or in the target dependent bits.


Index Nav: [Date Index] [Subject Index] [Author Index] [Thread Index]
Message Nav: [Date Prev] [Date Next] [Thread Prev] [Thread Next]