13 Nov
2013
13 Nov
'13
4:31 a.m.
Note that the safer rotate macro can be compiled into a rotate instruction just like the current macro. But LLVM and GCC don't know this. I've made requests to both compiler teams to implement this optimization, but I have no idea if they'll go for it.
http://llvm.org/bugs/show_bug.cgi?id=17904 http://gcc.gnu.org/bugzilla/show_bug.cgi?id=59100
John Regehr
On 11/12/2013 08:55 PM, John Regehr wrote:
#define ROTL32(n,x) (((x)<<(n)) | ((x)>>((-(n)&31))))
The problem is n==0, not n==32. So this fixes the undefined behavior:
#define ROTL32(n,x) ((n)==0?(x):(((x)<<(n)) | ((x)>>(32-(n)))))
John
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