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Date: Tue, 14 May 2013 14:31:00 -0000
From: Pedro Alves <palves@redhat.com>
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To: Tom Tromey <tromey@redhat.com>
CC: gdb-patches@sourceware.org
Subject: Re: [RFC, v2] Fix PR symtab/15391
References: <87hai57jsc.fsf@fleche.redhat.com>
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On 05/14/2013 02:50 PM, Tom Tromey wrote:
> +  if (TYPE_LENGTH (value_type (value)) < sizeof (LONGEST))
> +    {
> +      LONGEST nbits = 8 * TYPE_LENGTH (value_type (value));
> +      LONGEST m1 = -1;
> +
> +      if (((byte_offset >> (nbits - 1)) & 1) != 0)
> +	byte_offset |= m1 << nbits;
> +    }

I believe left-shifting a negative number is actually undefined
behavior.  It's more usual/idiomatic to do sign extension like:

LONGEST nbits = 8 * TYPE_LENGTH (value_type (value));
int signbit = 1U << nbits;
byte_offset = (byte_offset ^ signbit) - signbit;

-- 
Pedro Alves