Hi, sorry, for the delay. Sandra seems to be even more busy than me...
Attached is a combined patch of the original part 1, and the update, in diff -up format. On Mon, 11 Nov 2013 13:10:45, Richard Biener wrote: > > On Thu, Oct 31, 2013 at 1:46 AM, Sandra Loosemore > <san...@codesourcery.com> wrote: >> On 10/29/2013 02:51 AM, Bernd Edlinger wrote: >>> >>> >>> On Mon, 28 Oct 2013 21:29:24, Sandra Loosemore wrote: >>>> >>>> On 10/28/2013 03:20 AM, Bernd Edlinger wrote: >>>>> >>>>> I have attached an update to your patch, that should >>>>> a) fix the recursion problem. >>>>> b) restrict the -fstrict-volatile-bitfields to not violate the C++ >>>>> memory model. >> >> >> Here's a new version of the update patch. >> >> >>>> Alternatively, if strict_volatile_bitfield_p returns false but >>>> flag_strict_volatile_bitfields> 0, then always force to word_mode and >>>> change the -fstrict-volatile-bitfields documentation to indicate that's >>>> the fallback if the insertion/extraction cannot be done in the declared >>>> mode, rather than claiming that it tries to do the same thing as if >>>> -fstrict-volatile-bitfields were not enabled at all. >> >> >> I decided that this approach was more expedient, after all. >> >> I've tested this patch (in conjunction with my already-approved but >> not-yet-applied patch) on mainline for arm-none-eabi, x86_64-linux-gnu, and >> mips-linux gnu. I also backported the entire series to GCC 4.8 and tested >> there on arm-none-eabi and x86_64-linux-gnu. OK to apply? > > Hm, I can't seem to find the context for > > @@ -923,6 +935,14 @@ > store_fixed_bit_field (str_rtx, bitsize, bitnum, 0, 0, value); > return; > } > + else if (MEM_P (str_rtx) > + && MEM_VOLATILE_P (str_rtx) > + && flag_strict_volatile_bitfields> 0) > + /* This is a case where -fstrict-volatile-bitfields doesn't apply > + because we can't do a single access in the declared mode of the field. > + Since the incoming STR_RTX has already been adjusted to that mode, > + fall back to word mode for subsequent logic. */ > + str_rtx = adjust_address (str_rtx, word_mode, 0); > > /* Under the C++0x memory model, we must not touch bits outside the > bit region. Adjust the address to start at the beginning of the > > and the other similar hunk. I suppose they apply to earlier patches > in the series? I suppose the above applies to store_bit_field (diff -p > really helps!). Why would using word_mode be any good as > fallback? That is, why is "Since the incoming STR_RTX has already > been adjusted to that mode" not the thing to fix? > Well, this hunk does not force the access to be in word_mode. Instead it allows get_best_mode to choose the access to be in any mode from QI to word_mode. It is there to revert the effect of this weird code in expr.c (expand_assigment): if (volatilep && flag_strict_volatile_bitfields> 0) to_rtx = adjust_address (to_rtx, mode1, 0); Note that this does not even check if the access is on a bit-field ! The problem with the strict_volatile_bitfields is that it is used already before the code reaches store_bit_field or extract_bit_field. It starts in get_inner_reference, (which is not only used in expand_assignment and expand_expr_real_1) Then this, if (volatilep && flag_strict_volatile_bitfields> 0) op0 = adjust_address (op0, mode1, 0); and then this, /* If the field is volatile, we always want an aligned access. Do this in following two situations: 1. the access is not already naturally aligned, otherwise "normal" (non-bitfield) volatile fields become non-addressable. 2. the bitsize is narrower than the access size. Need to extract bitfields from the access. */ || (volatilep && flag_strict_volatile_bitfields> 0 && (bitpos % GET_MODE_ALIGNMENT (mode) != 0 || (mode1 != BLKmode && bitsize < GET_MODE_SIZE (mode1) * BITS_PER_UNIT))) As a result, a read access to an unaligned volatile data member does not even reach the expand_bit_field if flag_strict_volatile_bitfields <= 0, and instead goes through convert_move (target, op0, unsignedp). I still believe the proposed patch is guaranteed to not change anything if -fno-strict-volatile-bitfields is used, and even if we can not guarantee that it creates exactly the same code for cases where the strict-volatile-bitfields does not apply, it certainly generates valid code, where we had invalid code, or ICEs without the patch. OK for trunk? Bernd. > Richard. > >> -Sandra
patch-bitfields.diff
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