CODEWITHSUNDEEP
optimizationcudagpunvcc
Caian
Caian

Reputation: 491

Completely disable optimizations on NVCC

I'm trying to measure peak single-precision flops on my GPU, for that I'm modifying a PTX file to perform successive MAD instructions on registers. Unfortunately the compiler is removing all the code because it actually does nothing usefull since I do not perform any load/store of the data. Is there a compiler flag or pragma to add to the code so the compiler does not touch it?

Thanks.

Upvotes: 5

Views: 7116

Answers (5)

Andrei Pokrovsky
Andrei Pokrovsky

Reputation: 3836

These worked for me:

-g -G -Xcompiler -O0 -Xptxas -O0 -lineinfo -O0

Upvotes: 0

njuffa
njuffa

Reputation: 26085

To completely disable optimizations with nvcc, you can use the following:

nvcc -O0 -Xopencc -O0 -Xptxas -O0  // sm_1x targets using Open64 frontend
nvcc -O0 -Xcicc -O0 -Xptxas -O0 // sm_2x and sm_3x targets using NVVM frontend

Note that the resulting code may be extremely slow. The -O0 flag is passed to the host compiler to disable host code optimization. The -Xopencc -O0 and -Xcicc -O0 flags control the compiler frontend (the part that produces PTX) and turn off optimizations there. The -Xptxas -O0 flag controls the compiler backend (the part that converts PTX to machine code) and turns off optimizations in that part. Note that -Xopencc, -Xcicc, and -Xptxas flags are component-level flags, and unless documented in the nvcc manual, should be considered unsupported.

Upvotes: 9

CygnusX1
CygnusX1

Reputation: 21779

(I am still in CUDA 4.0, it may have changed with the new version)

To disable optimizations of ptxas (the tool that converts ptx into cubin) you need to pass an option --opt-level 0 (default is --opt-level 3). If you want to pass this option through nvcc you will need to prefix it with --ptxas-options.

Do note however, that ptxas does a lot of useful optimizations that --- when disabled --- may render your code even slower if not incorrect at all! For example, it does register allocation and tries to predict where is shared and where is global memory.

Upvotes: 1

Roger Dahl
Roger Dahl

Reputation: 15734

I don't think there is any way to turn off such optimization in the compiler. You can work around this by adding code to store your values and wrapping that code in a conditional statement that is always false. To make a conditional that the compiler can't determine to always be false, use at least one variable (not just constants).

Upvotes: 3

yyfn
yyfn

Reputation: 767

As far as I know, there is no compiler flag or pragma for that. but you can compute more and store less

Upvotes: -2

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