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Re: [PATCH 2/3] Simplify and inline get_uleb128 and get_sleb128

On 04/22/2014 07:55 AM, Mark Wielaard wrote:
> On Thu, 2013-12-12 at 11:06 -0800, Josh Stone wrote:
>> On 12/12/2013 04:13 AM, Petr Machata wrote:
>>> Josh Stone <> writes:
>>>> +#define get_sleb128_step(var, addr, nth)			  \
>>>>    do {							  \
>>>> +    unsigned char __b = *(addr)++;				  \
>>>> +    if (likely ((__b & 0x80) == 0))				  \
>>>>        {							  \
>>>> +	   struct { signed int i:7; } __s = { .i = __b };	  \
>>>> +	   (var) |= (typeof (var)) __s.i << ((nth) * 7);	  \
>>> Oh, the bitfield trick is clever!
>> I should give credit, I found that trick here:
>> The former code was trying to sign-extend after the value was shifted
>> and combined, which as a variable width is harder.  I really like that
>> this way the compiler is fully aware that this is a sign extension,
>> rather than being a side effect of ORing bits or left-right shifts.
> Sadly the neat trick triggers undefined behavior since we are trying to
> left shift a negative value. Even though it appears to work currently I
> am slightly afraid a compiler optimization might take advantage of this
> in the future (since it is undefined behavior it could just assume
> negative values won't occur) especially since this code is inlined in a
> lot of places, causing hard to diagnose errors.
> The attached patch is very much not clever, but does what is intended in
> a well-defined way (it is basically what the DWARF spec gives as pseudo
> code). Does anybody see a better way?

Multiply instead of shift.  I.e.

  (typeof (var)) (__b & 0x7f) * (1 << ((nth) * 7));

With any luck the compiler will even notice it's a multiply by a value with one
bit set, and convert it back to a shift during optimization.


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