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Re: [PATCH] Clear upper bits during sign extension
- From: Doug Evans <dje at google dot com>
- To: Yao Qi <yao at codesourcery dot com>
- Cc: Joel Brobecker <brobecker at adacore dot com>, gdb-patches <gdb-patches at sourceware dot org>
- Date: Sun, 28 Dec 2014 21:29:22 -0800
- Subject: Re: [PATCH] Clear upper bits during sign extension
- Authentication-results: sourceware.org; auth=none
- References: <1419815569-21854-1-git-send-email-yao at codesourcery dot com> <20141229030659 dot GB2123 at adacore dot com> <87d273w2gl dot fsf at codesourcery dot com>
On Sun, Dec 28, 2014 at 7:37 PM, Yao Qi <yao@codesourcery.com> wrote:
> Joel Brobecker <brobecker@adacore.com> writes:
>
>> I think you can simplify the above with just:
>>
>> value &= ((LONGEST) 1 << bit) - 1;
>>
>> ? I don't know if the cast to LONGEST is really necessary, but we use
>> it for signbit, so I did the same for the mask...
>
> Yeah, that is better, or even we can left shift signbit by one.
>
> --
> Yao (éå)
>
> gdb:
>
> 2014-12-29 Yao Qi <yao@codesourcery.com>
>
> * utils.c (gdb_sign_extend): Clear bits from BIT in VALUE.
>
> diff --git a/gdb/utils.c b/gdb/utils.c
> index 47adb67..61873f7 100644
> --- a/gdb/utils.c
> +++ b/gdb/utils.c
> @@ -3032,6 +3032,9 @@ gdb_sign_extend (LONGEST value, int bit)
> {
> LONGEST signbit = ((LONGEST) 1) << (bit - 1);
>
> + /* Clear upper bits from bit BIT. */
> + value &= (signbit << 1) - 1;
> +
> value = (value ^ signbit) - signbit;
> }
It's not immediately clear to this reader that undefined behaviour is
avoided here.
E.g., what if sizeof (LONGEST) == 8 && bit == 64.
There's an assert at the beginning of the function that bit could be 64.