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Re: patch to fix constant math -5th patch, rtl
- From: Richard Sandiford <rdsandiford at googlemail dot com>
- To: Richard Biener <richard dot guenther at gmail dot com>
- Cc: Kenneth Zadeck <zadeck at naturalbridge dot com>, Mike Stump <mikestump at comcast dot net>, gcc-patches <gcc-patches at gcc dot gnu dot org>, Lawrence Crowl <crowl at google dot com>, Ian Lance Taylor <iant at google dot com>
- Date: Wed, 24 Apr 2013 13:44:59 +0100
- Subject: Re: patch to fix constant math -5th patch, rtl
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Richard Biener <firstname.lastname@example.org> writes:
> Can we in such cases please to a preparatory patch and change the
> CONST_INT/CONST_DOUBLE paths to do an explicit [sz]ext to
> mode precision first?
I'm not sure what you mean here. CONST_INT HWIs are already sign-extended
from mode precision to HWI precision. The 8-bit value 0xb10000000 must be
represented as (const_int -128); nothing else is allowed. E.g. (const_int 128)
is not a valid QImode value on BITS_PER_UNIT==8 targets.
> What does wide-int do with VOIDmode mode inputs?
> It seems to ICE on them for from_rtx and use garbage (0) for from_shwi. Ugh.
ICEing is right. As mentioned before, every rtx constant has a mode,
whether it's stored in the rtx or not. Callers must keep track of
what that mode is.
> Btw, plus_constant asserts that mode is either VOIDmode (I suppose
> semantically do "arbitrary precision")
No, not arbitrary precision. It's always the precision specified
by the "mode" parameter. The assert is:
gcc_assert (GET_MODE (x) == VOIDmode || GET_MODE (x) == mode);
This is because GET_MODE always returns VOIDmode for CONST_INT and
CONST_DOUBLE integers. The mode parameter is needed to tell us what
precision those CONST_INTs and CONST_DOUBLEs actually have, because
the rtx itself doesn't tell us. The mode parameter serves no purpose
So if the rtx does specify a mode (everything except CONST_INT and
CONST_DOUBLE), the assert is making sure that the caller has correctly
tracked the rtx's mode and provided the right mode parameter. The caller
must do that for all rtxes, it's just that we can't assert for it in the
CONST_INT and CONST_DOUBLE case, because the rtx has no mode to check
against. If CONST_INT and CONST_DOUBLE did have a mode to check against,
there would be no need for the mode parameter at all. Likewise there
would be no need for wide_int::from_rtx to have a mode parameter.