------- Additional Comments From schlie at comcast dot net 2004-10-20 18:34 -------
Subject: Re: wrong code emitted
Hi Andrew,
Please correct the failure mode description to " critical, wrong code",
as upon reviewing the avr.md file, it's now clear that this isn't a
"missed optimization"; GCC is failing to properly emit the instruction
explicitly matching it's specified QI mode operand constraints as specified
by a corresponding identically constrained C statement; more specifically:
(this behavior should be deterministic, and relies on no optimizations)
The explicit C declaration statement (which also fails for %):
signed char ss = (signed char)s / (signed char)s ;
Incorrectly emits an HI vs. QI mode instruction as specified avr.md
//uses <__divmodhi4> wrong, should be 8-bit <__divmodqi4>
ce: 99 81 ldd r25, Y+1 ; 0x01
d0: 89 81 ldd r24, Y+1 ; 0x01
d2: 28 2f mov r18, r24
d4: 33 27 eor r19, r19
d6: 27 fd sbrc r18, 7
d8: 30 95 com r19
da: 89 2f mov r24, r25
dc: 99 27 eor r25, r25
de: 87 fd sbrc r24, 7
e0: 90 95 com r25
e2: b9 01 movw r22, r18
e4: 0e 94 81 01 call __divmodhi4
e8: 6b 83 std Y+3, r22 ; 0x03
Where the corresponding avr.md specifications are:
(define_expand "divmodqi4"
[(set (reg:QI 24) (match_operand:QI 1 "register_operand" ""))
(set (reg:QI 22) (match_operand:QI 2 "register_operand" ""))
(parallel [(set (reg:QI 24) (div:QI (reg:QI 24) (reg:QI 22)))
(set (reg:QI 25) (mod:QI (reg:QI 24) (reg:QI 22)))
(clobber (reg:QI 22))
(clobber (reg:QI 23))])
(set (match_operand:QI 0 "register_operand" "") (reg:QI 24))
(set (match_operand:QI 3 "register_operand" "") (reg:QI 25))]
""
"")
(define_insn "*divmodqi4_call"
[(set (reg:QI 24) (div:QI (reg:QI 24) (reg:QI 22)))
(set (reg:QI 25) (mod:QI (reg:QI 24) (reg:QI 22)))
(clobber (reg:QI 22))
(clobber (reg:QI 23))]
""
"%~call __divmodqi4"
[(set_attr "type" "xcall")
(set_attr "cc" "clobber")])
(define_expand "divmodhi4"
[(set (reg:HI 24) (match_operand:HI 1 "register_operand" ""))
(set (reg:HI 22) (match_operand:HI 2 "register_operand" ""))
(parallel [(set (reg:HI 22) (div:HI (reg:HI 24) (reg:HI 22)))
(set (reg:HI 24) (mod:HI (reg:HI 24) (reg:HI 22)))
(clobber (reg:HI 26))
(clobber (reg:QI 21))])
(set (match_operand:HI 0 "register_operand" "") (reg:HI 22))
(set (match_operand:HI 3 "register_operand" "") (reg:HI 24))]
""
"")
(define_insn "*divmodhi4_call"
[(set (reg:HI 22) (div:HI (reg:HI 24) (reg:HI 22)))
(set (reg:HI 24) (mod:HI (reg:HI 24) (reg:HI 22)))
(clobber (reg:HI 26))
(clobber (reg:QI 21))]
""
"%~call __divmodhi4"
[(set_attr "type" "xcall")
(set_attr "cc" "clobber")])
Which seems correct, but for some reason GCC is not able to recognize
when it's lhs operand is a QI mode operand when attempting to match
this specification when emitting code?
Thanks again, -paul-
> From: pinskia at gcc dot gnu dot org <[EMAIL PROTECTED]>
> Reply-To: <[EMAIL PROTECTED]>
> Date: 19 Oct 2004 20:39:44 -0000
> To: <[EMAIL PROTECTED]>
> Subject: [Bug target/18065] wrong built-in functions selected
>
>
> ------- Additional Comments From pinskia at gcc dot gnu dot org 2004-10-19
> 20:39 -------
> char ss = s % s ; //<__divmodhi4> wrong, should be 8-bit <__divmodqi4>
> This is not wrong as we sign extend the arguments as required by the C
> standard (to int).
>
> So this is just an missed-optimization.
>
> --
> What |Removed |Added
> ----------------------------------------------------------------------------
> Severity|critical |minor
> Component|c |target
> Keywords| |missed-optimization
>
>
> http://gcc.gnu.org/bugzilla/show_bug.cgi?id=18065
>
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--
http://gcc.gnu.org/bugzilla/show_bug.cgi?id=18065
