[LLVMdev] [cfe-dev] Disable Short-Circuit Evaluation?
David A. Greene
greened at obbligato.org
Mon Oct 10 07:29:48 PDT 2011
Justin Holewinski <justin.holewinski at gmail.com> writes:
> int globalIndexY2 = get_group_id(1)*186 + 6*get_local_id(1) + 2 + 1;
> bool valid2 = validX && globalIndexY2 >= 4 && globalIndexY2 < 3910;
> Clang, even at -O0, is performing short-circuit evaluation of these
> expressions, resulting in a fair number of branch instructions being
It has to. This is the semantics of C. Short-circuiting is used to
defend against all sorts of undefined behavior in real code.
> For most targets, this is a beneficial optimizations.
For all targets. If the code doesn't work, it's pretty useless. :)
> However, for my target (PTX), it would be most beneficial to actually
> evaluate the entire expression and remove the unneeded branches. Is
> this possible with current Clang/LLVM?
So for PTX what you want is if-conversion. I believe there is a pass
that does this in the ARM codegen. Of course the PTX backend will have
to support mask bits. I don't know if it does currently.
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