Re: [PATCH IRA] update_equiv_regs fails to set EQUIV reg-note for pseudo with more than one definition

["Bin.Cheng" <amker dot cheng at gmail dot com>]

On Wed, Feb 4, 2015 at 12:28 AM, Jeff Law <law@redhat.com> wrote:
> On 02/03/15 01:29, Bin.Cheng wrote:
>>
>>
>> Hmm, if I understand correctly, it's a code size regression, so I
>> don't think it's appropriate to adapt the test case.  Either the patch
>> or something else in GCC is doing wrong, right?
>>
>> Hi Alex, could you please file a PR with full dump information for
>> tracking?
>
> But if the code size regression is due to the older compiler incorrectly
> handling the promotion of REG_EQUAL to REG_EQUIV notes, then the test
> absolutely does need updating as the codesize was dependent on incorrect
> behaviour in the compiler.

Hi Jeff,

I looked into the test and can confirm the previous compilation is correct.
The cover letter of this patch said IRA mis-handled REQ_EQUIV before,
but in this case it is REG_EQUAL that is lost.  The full dump (without
this patch) after IRA is like:

   10: NOTE_INSN_BASIC_BLOCK 2
    2: r116:SI=r0:SI
    3: r117:SI=r1:SI
      REG_DEAD r1:SI
    4: r118:SI=r2:SI
      REG_DEAD r2:SI
    5: NOTE_INSN_FUNCTION_BEG
   12: r2:SI=0x1
   13: r1:SI=0
   15: r0:SI=call [`lseek'] argc:0
      REG_DEAD r2:SI
      REG_DEAD r1:SI
      REG_CALL_DECL `lseek'
   16: r111:SI=r0:SI
      REG_DEAD r0:SI
   17: r2:SI=0x2
   18: r1:SI=0
   19: r0:SI=r116:SI
      REG_DEAD r116:SI
   20: r0:SI=call [`lseek'] argc:0
      REG_DEAD r2:SI
      REG_DEAD r1:SI
      REG_CALL_DECL `lseek'
   21: r112:SI=r0:SI
      REG_DEAD r0:SI
   22: cc:CC=cmp(r111:SI,0xffffffffffffffff)
   23: pc={(cc:CC==0)?L46:pc}
      REG_DEAD cc:CC
      REG_BR_PROB 159
   24: NOTE_INSN_BASIC_BLOCK 3
   25: cc:CC=cmp(r112:SI,0xffffffffffffffff)
   26: pc={(cc:CC==0)?L50:pc}
      REG_DEAD cc:CC
      REG_BR_PROB 159
   27: NOTE_INSN_BASIC_BLOCK 4
   28: NOTE_INSN_DELETED
   29: {cc:CC_NOOV=cmp(r112:SI-r111:SI,0);r114:SI=r112:SI-r111:SI;}
      REG_DEAD r112:SI
   30: pc={(cc:CC_NOOV==0)?L54:pc}
      REG_DEAD cc:CC_NOOV
      REG_BR_PROB 400
   31: NOTE_INSN_BASIC_BLOCK 5
   32: [r117:SI]=r111:SI
      REG_DEAD r117:SI
      REG_DEAD r111:SI
   33: [r118:SI]=r114:SI
      REG_DEAD r118:SI
      REG_DEAD r114:SI
    7: r110:SI=0
      REG_EQUAL 0
   76: pc=L34
   77: barrier
   46: L46:
   45: NOTE_INSN_BASIC_BLOCK 6
    8: r110:SI=r111:SI
      REG_DEAD r111:SI
      REG_EQUAL 0xffffffffffffffff
   78: pc=L34
   79: barrier
   50: L50:
   49: NOTE_INSN_BASIC_BLOCK 7
    6: r110:SI=r112:SI
      REG_DEAD r112:SI
      REG_EQUAL 0xffffffffffffffff
   80: pc=L34
   81: barrier
   54: L54:
   53: NOTE_INSN_BASIC_BLOCK 8
    9: r110:SI=0xffffffffffffffff
      REG_EQUAL 0xffffffffffffffff
   34: L34:
   35: NOTE_INSN_BASIC_BLOCK 9
   40: r0:SI=r110:SI
      REG_DEAD r110:SI
   41: use r0:SI

Before r216169 (with REG_EQUAL in insn9), jumps from basic block 6/7/8
-> 9 can be merged because r110 equals to -1 afterwards.  But with the
patch, the equal information of r110==-1 in basic block 8 is lost.  As
a result, jump from 8->9 can't be merged and two additional
instructions are generated.

I suppose the REG_EQUAL note is correct in insn9?  According to
GCCint, it only means r110 set by insn9 will be equal to the value at
run time at the end of this insn but not necessarily elsewhere in the
function.

I also found another problem (or mis-leading?) with the document:
"Thus, compiler passes prior to register allocation need only check
for REG_EQUAL notes and passes subsequent to register allocation need
only check for REG_EQUIV notes".  This seems not true now as in this
example, passes after register allocation do take advantage of
REG_EQUAL in optimization and we can't achieve that by using
REG_EQUIV.

Thanks,
bin

>
> jeff