[Bug middle-end/81444] New: expmed.c:init_expmed_one_mode uses wrong mode for widening cost computations

["gjl at gcc dot gnu.org" <gcc-bugzilla at gcc dot gnu dot org>]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=81444

            Bug ID: 81444
           Summary: expmed.c:init_expmed_one_mode uses wrong mode for
                    widening cost computations
           Product: gcc
           Version: 7.1.1
            Status: UNCONFIRMED
          Severity: normal
          Priority: P3
         Component: middle-end
          Assignee: unassigned at gcc dot gnu.org
          Reporter: gjl at gcc dot gnu.org
  Target Milestone: ---

expmed.c:init_expmed_one_mode uses a wrong mode for widening cost computations.
 For example, on avr this function will compute the costs for
set_mul_highpart_cost as follows:

Almost correct (TImode is strange, this is due to init_expmed_one_conv
clobbering modes):

init_expmed_one_mode[:pass=?]: 0, (truncate:QI (lshiftrt:HI (mult:HI
(zero_extend:HI (reg:TI 42))
            (zero_extend:HI (reg:TI 42)))
        (const_int 8 [0x8]))) = 20

Wrong: mult_highpart will use SImode (2*HImode).  This target is special as it
provides a 24-bit mode (PSI) between HI and SI.  The cost of "-88" will be used
in the remainder where (truncate:HI (lshiftrt:SI (mult:SI... is the actual RTX.

init_expmed_one_mode[:pass=?]: 0, (truncate:HI (lshiftrt:PSI (mult:PSI
(zero_extend:PSI (reg:TI 42))
            (zero_extend:PSI (reg:TI 42)))
        (const_int 16 [0x10]))) = -88

Following is not used.  It would be unusable for sensible widening operation
because SI <  2 * PSI.

init_expmed_one_mode[:pass=?]: 0, (truncate:PSI (lshiftrt:SI (mult:SI
(zero_extend:SI (reg:TI 42))
            (zero_extend:SI (reg:TI 42)))
        (const_int 24 [0x18]))) = 32

Correct again but still strange TImode:

init_expmed_one_mode[:pass=?]: 0, (truncate:SI (lshiftrt:DI (mult:DI
(zero_extend:DI (reg:TI 42))
            (zero_extend:DI (reg:TI 42)))
        (const_int 32 [0x20]))) = 64

...

Instead, what expmed computes costs for should read:

init_expmed_one_mode[:pass=?]: 0, (truncate:QI (lshiftrt:HI (mult:HI
(zero_extend:HI (reg:QI 42))
            (zero_extend:HI (reg:QI 42)))
        (const_int 8 [0x8]))) = 20

init_expmed_one_mode[:pass=?]: 0, (truncate:HI (lshiftrt:SI (mult:SI
(zero_extend:SI (reg:HI 42))
            (zero_extend:SI (reg:HI 42)))
        (const_int 16 [0x10]))) = 32

init_expmed_one_mode[:pass=?]: 0, (truncate:SI (lshiftrt:DI (mult:DI
(zero_extend:DI (reg:SI 42))
            (zero_extend:DI (reg:SI 42)))
        (const_int 32 [0x20]))) = 64

etc.