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Re: Problem with init of structure bit fields
Stelian Pop <stelian@popies.net> writes:
> On Wed, Jun 03, 2009 at 07:49:29PM +0200, Stelian Pop wrote:
>
>> I'm doing a port of gcc 4.3.3 on a custom architecture and I'm having trouble
>> when initializing the bit fields of a structure.
>
> Ok, after further analysis, it looks like a genuine bug in gcc,
> in store_bit_field_1(): when a field is bigger than a word, the
> rvalue is splitted in several words, after being placed in a
> smallest_mode_for_size() operand.
>
> But the logic for adressing the words of this operand is buggy
> in the WORDS_BIG_ENDIAN case, the most signficant words being
> used instead of the least significant ones.
>
> The following patch corrects this, and makes the
> gcc.c-torture/execute/991118-1.c testcase work correctly on my platform.
>
> Stelian.
>
> diff --git a/gcc/expmed.c b/gcc/expmed.c
> index dc61de7..03c60a8 100644
> --- a/gcc/expmed.c
> +++ b/gcc/expmed.c
> @@ -582,7 +582,10 @@ store_bit_field_1 (rtx str_rtx, unsigned HOST_WIDE_INT bitsize,
> {
> /* If I is 0, use the low-order word in both field and target;
> if I is 1, use the next to lowest word; and so on. */
> - unsigned int wordnum = (backwards ? nwords - i - 1 : i);
> + unsigned int wordnum = (backwards
> + ? GET_MODE_SIZE(fieldmode) / UNITS_PER_WORD
> + - i - 1
> + : i);
> unsigned int bit_offset = (backwards
> ? MAX ((int) bitsize - ((int) i + 1)
> * BITS_PER_WORD,
Your patch looks correct. However, it makes me wonder how the test case
passes on existing big-endian platforms, such as the PowerPC. Can you
explain how that works?
Ian