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On 04/07/11 04:47, Bernd Schmidt wrote:
> PR47976 is a followup to PR47166; the patch there caused this problem.
>
> The problem occurs in reload. There are two autoinc addresses which
> inherit from one another, and we delete an insn that is necessary.
>
> We reach this code when reloading the second autoinc address:
>
> 6821 if (optimize && REG_P (oldequiv)
> 6822 && REGNO (oldequiv) < FIRST_PSEUDO_REGISTER
> 6823 && spill_reg_store[REGNO (oldequiv)]
> 6824 && REG_P (old)
> (gdb)
> 6825 && (dead_or_set_p (insn,
> 6826 spill_reg_stored_to[REGNO (oldequiv)])
> 6827 || rtx_equal_p (spill_reg_stored_to[REGNO (oldequiv)],
> 6828 old)))
> 6829 delete_output_reload (insn, j, REGNO (oldequiv), reloadreg);
>
> reload_inherited[j] is 1 at this point, so oldequiv == reloadreg.
>
> (gdb) p debug_rtx (spill_reg_store[7])
> (insn 719 718 232 10 (set (reg:SI 7 r7)
> (reg:SI 3 r3 [orig:339 ivtmp.79 ] [339])) -1 (nil))
> (gdb) p debug_rtx (spill_reg_stored_to[7])
> (reg:SI 3 r3)
>
> Prior to the PR47166 patch, we had spill_reg_store[7] equal to insn 718,
> which doesn't involve register 7 at all:
>
> (insn 718 221 719 10 (set (reg:SI 3 r3 [orig:339 ivtmp.79 ] [339])
> (plus:SI (reg:SI 3 r3 [orig:339 ivtmp.79 ] [339])
> (const_int 8 [0x8]))) 4 {*arm_addsi3} (nil))
>
> That was sufficient to generate enough confusion to make the compiler
> think it couldn't delete the output reload.
>
> I think the problem is simply that the (set (r7) (r3)) is the opposite
> direction of a normal spill_reg_store - normally you write a spill reg
> to its destination, but autoinc reloads are somewhat special.
>
> If delete_output_reload isn't valid for (at least some) autoincs, we can
> simply not record them in spill_reg_store. That's part of the patch
> below; it seems to fix the problem. I've also deleted the code quoted
> above since it's pointless to have reload deleting dead stores to
> registers: that's what DCE is for. I've observed no code generation
> changes other than for the testcase from either of these changes, with
> both an ARM and an sh compiler.
>
> Comments?
Looks good to me. I like letting DCE do its job, particularly if it
allows us to even trivially simplify this code ;-)
jeff
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