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RE: [PATCH] reimplement -fstrict-volatile-bitfields v4, part 1/2
- From: Bernd Edlinger <bernd dot edlinger at hotmail dot de>
- To: Richard Biener <richard dot guenther at gmail dot com>
- Cc: Sandra Loosemore <sandra at codesourcery dot com>, GCC Patches <gcc-patches at gcc dot gnu dot org>, Julian Brown <julian at codesourcery dot com>
- Date: Fri, 15 Nov 2013 14:24:00 +0100
- Subject: RE: [PATCH] reimplement -fstrict-volatile-bitfields v4, part 1/2
- Authentication-results: sourceware.org; auth=none
- References: <52463CA4 dot 7060303 at codesourcery dot com>,<CAFiYyc1z=BVSME0317W7HVr9HAvZ0PvKsdT9Yx-vsmNgEjE52g at mail dot gmail dot com>,<526161EC dot 5070200 at codesourcery dot com>,<325c359b-22e1-43dc-8050-5a11deb66e95 at email dot android dot com>,<52649035 dot 6000802 at codesourcery dot com>,<DUB122-W9C01556B8690D446546A6E4080 at phx dot gbl>,<526F2B94 dot 8020907 at codesourcery dot com>,<DUB122-W1396BF39760E97A090742BE4090 at phx dot gbl>,<5271A851 dot 6000808 at codesourcery dot com>,<CAFiYyc3bYPF5xSXSSq-3ZU-dT5Ou00x=qC-u5sXxkZVf3tH86g at mail dot gmail dot com>,<DUB122-W25E69525F3AA97742A1188E4F80 at phx dot gbl>,<CAFiYyc2af8kUuAJ3ocF+N+=Fa-HCr-nOPDgB_0BpVYaNxb-O3w at mail dot gmail dot com>,<DUB122-W156FAE71D5E58C1366D1D1E4FB0 at phx dot gbl>,<CAFiYyc2A-he96v+T-5Mg9ro8Lx_rrP_NK5OwJqJiMQ3UF+_7SA at mail dot gmail dot com>,<DUB122-W414FF92FA702FE3EDA09D9E4FB0 at phx dot gbl>,<CAFiYyc2JLZndiu+x9DgrGczWQcZWiAOawFiJ5_vgmH3FXm=T=w at mail dot gmail dot com>
>
> But then why is the mode QImode in the first place? The access is
> definitely of SImode.
>
that's in the test case:
s->arr[0] = 0x12345678;
it is QImode from that in expand_assignment:
to_rtx = expand_expr (tem, NULL_RTX, VOIDmode, EXPAND_WRITE);
tem is "s", a MEM_REF, of QImode, perfectly aligned. the mode is only
OK to access *s or s->pad. It is wrong for s->srr[0].
in store_bit_field the mode is used in store_fixed_bit_field:
mode = GET_MODE (op0);
if (GET_MODE_BITSIZE (mode) == 0
|| GET_MODE_BITSIZE (mode)> GET_MODE_BITSIZE (word_mode))
mode = word_mode;
mode = get_best_mode (bitsize, bitnum, bitregion_start, bitregion_end,
MEM_ALIGN (op0), mode, MEM_VOLATILE_P (op0));
if (mode == VOIDmode)
goto boom;
so this restricts the possible access mode. word_mode, means no restriction.
Everything would be OK if MEM_ALIGN(op0) was byte-aligned, but we have
a problem if MEM_ALIGN(op0)>=WORD_MODE.
Do you understand?
Bernd.