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Re: [PATCH] Fix big-endian bswap
- From: Richard Biener <richard dot guenther at gmail dot com>
- To: Wilco Dijkstra <Wilco dot Dijkstra at arm dot com>
- Cc: GCC Patches <gcc-patches at gcc dot gnu dot org>, nd <nd at arm dot com>
- Date: Tue, 28 Jun 2016 15:37:01 +0200
- Subject: Re: [PATCH] Fix big-endian bswap
- Authentication-results: sourceware.org; auth=none
- References: <HE1PR0801MB1482D3151EBD44465B472D46832D0 at HE1PR0801MB1482 dot eurprd08 dot prod dot outlook dot com>
On Thu, Jun 23, 2016 at 6:34 PM, Wilco Dijkstra <Wilco.Dijkstra@arm.com> wrote:
> This patch fixes a bug in the bswap pass. In big-endian BIT_FIELD_REF uses
> big-endian bit numbering so we need to adjust the bit position.
> The existing version could potentially generate incorrect code however GCC doesn't
> emit a BIT_FIELD_REF to access the low byte in a register, so the symbolic number
> never matches in big-endian.
>
> The test gcc.dg/optimize-bswapsi-4.c now passes on AArch64, no other changes.
> OK for commit?
Ok.
Thanks,
Richard.
> ChangeLog:
> 2016-06-23 Wilco Dijkstra <wdijkstr@arm.com>
>
> * gcc/tree-ssa-math-opts.c (find_bswap_or_nop_1): Adjust bitnumbering
> for big-endian BIT_FIELD_REF.
>
> --
> diff --git a/gcc/tree-ssa-math-opts.c b/gcc/tree-ssa-math-opts.c
> index 513ef0b3f4eb29a35eae8a0eb14ee8f8c24fcfd9..d31c12fd818a713ca3d251b9464015b147235bbe 100644
> --- a/gcc/tree-ssa-math-opts.c
> +++ b/gcc/tree-ssa-math-opts.c
> @@ -2307,6 +2307,10 @@ find_bswap_or_nop_1 (gimple *stmt, struct symbolic_number *n, int limit)
> && bitsize % BITS_PER_UNIT == 0
> && init_symbolic_number (n, TREE_OPERAND (rhs1, 0)))
> {
> + /* Handle big-endian bit numbering in BIT_FIELD_REF. */
> + if (BYTES_BIG_ENDIAN)
> + bitpos = TYPE_PRECISION (n->type) - bitpos - bitsize;
> +
> /* Shift. */
> if (!do_shift_rotate (RSHIFT_EXPR, n, bitpos))
> return NULL;
>