llvm dev - Oct 2011 - [LLVMdev] [cfe-dev] Disable Short-Circuit Evaluation?

Is there any way to disable short-circuit evaluation of expressions in
Clang/LLVM?

Let's say I have C code like the following:

bool validX = get_group_id(0) > 32;

int globalIndexY0 = get_group_id(1)*186 + 6*get_local_id(1) + 0 + 1;
bool valid0       = validX && globalIndexY0 >= 4 &&
globalIndexY0 < 3910;

int globalIndexY1 = get_group_id(1)*186 + 6*get_local_id(1) + 1 + 1;
bool valid1       = validX && globalIndexY1 >= 4 &&
globalIndexY1 < 3910;

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
generated.  For most targets, this is a beneficial optimizations.  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?

-- 

Thanks,

Justin Holewinski
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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
> generated.
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.

                                -Dave

A compilable testcase:


extern int get_group_id (int);
extern int get_local_id (int);

extern void check (bool, bool, bool);

void
foo (void)
{
   bool validX = get_group_id (0) > 32;

   int globalIndexY0 = get_group_id (1) * 186 + 6 * get_local_id (1) + 0 + 1;
   bool valid0 = validX && globalIndexY0 >= 4 &&
globalIndexY0 < 3910;

   int globalIndexY1 = get_group_id (1) * 186 + 6 * get_local_id (1) + 1 + 1;
   bool valid1 = validX && globalIndexY1 >= 4 &&
globalIndexY1 < 3910;

   int globalIndexY2 = get_group_id (1) * 186 + 6 * get_local_id (1) + 2 + 1;
   bool valid2 = validX && globalIndexY2 >= 4 &&
globalIndexY2 < 3910;

   check (valid0, valid1, valid2);
}

10.10.2011, 18:29, "David A. Greene" <greened at
obbligato.org>:
> 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
>>  generated.
>
> It has to.  This is the semantics of C.  Short-circuiting is used to
> defend against all sorts of undefined behavior in real code.
More precisely, && and || are sequence points (though in C++ they may
not be
sequence points if respective operator is overloaded)

[1] http://en.wikipedia.org/wiki/Sequence_point

-- 
Regards,
Konstantin