[LLVMdev] Implementing a new feature in LVVM

[arsene.perard at laposte.net]

Hello everyone,

I'm relatively new to LLVM and I would like to implement a feature, so I want your advice on its feasibility and difficulty. I want the code generator to support such code :

%1 = ; <4 x float> vector
%2 = ; <4 x float> vector
%3 = fcmp ogt <4 x float> %1, %2
%4 = bitcast <4 x i1> %3 to i4
%5 = zext i4 %4 to i32

and generate a movmskps instruction (correct me if it's not possible). So the code generator would have to find such pattern (fcmp followed by a bitcast and/or zext) and generate correct assembly (using movmskps or a slower code path). Currently, LLVM 3.3 seems to generate wrong code (see bug 17479 : http://llvm.org/bugs/show_bug.cgi?id=17479).

I've looked a little through the code and in particular in the selection dag for x86, but I don't know where to start.

Thank you in advance,

Arsène PERARD-GAYOT.

Une messagerie gratuite, garantie à vie et des services en plus, ça vous tente ?
Je crée ma boîte mail www.laposte.net