Is inline PTX assembly code powerful?

Question

I saw some code samples where people use inline PTX assembly code in C code. Doc in CUDA toolkit mentions that PTX is powerful, why is it so? What advantage we get if we use such codes in our C code?

Answer 1

Inline PTX gives you access to instructions not exposed via CUDA intrinsincs, and lets you apply optimizations that are either lacking in the compiler or prohibited by language specifications. For a worked example where use of inline PTX is advantageous, see: 128 bit integer on cuda?

The 128-bit addition using inline PTX requires just four instructions, since it has direct access to the carry flag. As a HLL, C/C++ does not have a representation for a carry flag, as a given hardware platform may have no carry flag (e.g. MIPS), a single carry flag (e.g. x86, sm_2x), or even multiple carry flags. In contrast to the 4-instruction PTX versions of 128-bit addition and subtraction, these operations might be coded in C as follows:

#define SUBCcc(a,b,cy,t0,t1,t2) \
  (t0=(b)+cy, t1=(a), cy=t0<cy, t2=t1<t0, cy=cy+t2, t1-t0)
#define SUBcc(a,b,cy,t0,t1) \
  (t0=(b), t1=(a), cy=t1<t0, t1-t0)
#define SUBC(a,b,cy,t0,t1) \
  (t0=(b)+cy, t1=(a), t1-t0)
#define ADDCcc(a,b,cy,t0,t1) \
  (t0=(b)+cy, t1=(a), cy=t0<cy, t0=t0+t1, t1=t0<t1, cy=cy+t1, t0=t0)
#define ADDcc(a,b,cy,t0,t1) \
  (t0=(b), t1=(a), t0=t0+t1, cy=t0<t1, t0=t0)
#define ADDC(a,b,cy,t0,t1) \
  (t0=(b)+cy, t1=(a), t0+t1)

unsigned int cy, t0, t1, t2;

res.x = ADDcc  (augend.x, addend.x, cy, t0, t1);
res.y = ADDCcc (augend.y, addend.y, cy, t0, t1);
res.z = ADDCcc (augend.z, addend.z, cy, t0, t1);
res.w = ADDC   (augend.w, addend.w, cy, t0, t1);

res.x = SUBcc  (minuend.x, subtrahend.x, cy, t0, t1);
res.y = SUBCcc (minuend.y, subtrahend.y, cy, t0, t1, t2);
res.z = SUBCcc (minuend.z, subtrahend.z, cy, t0, t1, t2);
res.w = SUBC   (minuend.w, subtrahend.w, cy, t0, t1);

The addition and subtraction above probably compile to about three to four times the number of instructions used by the corresponding inline PTX version.