On 04/22/2014 08:01 AM, Richard Henderson wrote:
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 <jistone(a)redhat.com> 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:
>>
http://graphics.stanford.edu/~seander/bithacks.html#FixedSignExtend
>>
>> 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.
The trick in the original was that "(typeof (var)) __s.i" would extend
the sign from the 7th bit, which I don't your mask will do. But if the
only UB is left-shifting negative, then I think we can combine them.
Bah. That 1 needs to be typeof (var) too.
So in total:
struct { signed int i:7; } __s = { .i = __b };
(var) |= (typeof (var)) __s.i * ((typeof (var)) 1 << ((nth) * 7));
Better?