Almost into passable frame rate territory. Next version could be business card VR:)
Revisiting the Business Card Raytracer
31–40 of 54 posts
Re: Revisiting the Business Card Raytracer
#32[deleted]
Re: Revisiting the Business Card Raytracer
#33Nice work on the GPU programming, and the multicore before that, but I'm mystified why going from -O0 to -O3 is named an "optimisation". All respect for Fabien, but running code that's supposed to run faster than a snail (and if you're not debugging and require -O0 for reasonable output) implies -O2 or -O3. (In practice, -O3 often doesn't give much performance over -O2, despite increasing compile times.) The initial…
Agree with you
Re: Revisiting the Business Card Raytracer
#34> Opening the binary with Binary Ninja revealed that clang had already managed to leverage the SSE registers. X86-64 uses SSE registers for all floating point operations. I'm not sure that the author realized that they were looking at an -O0 binary. -O0 does not do vectorization (or anything else for that matter).
Looking at it on Godbolt, it doesn't really leverage SSE on -O3, either. You can get a reasonable grasp of whether it's using SSE effectively or not just by looking at the instruction names. mulss: multiplication of a single single-precision floating point value. mulsd: multiplication of a single double-precision floating point value. mulps: multiplication of a packed group of single-precision floating point values.…
This post is really topical for me. I spent hours yesterday trying to write explicit SIMD code[0] for my the vector dot product in my raytracer and all I managed to do was slow the code down about 20-30%.
The code generated by Rust from the naive solution uses ss instructions mostly whereas my two tries using `mm_dp_ps` and `mm_mul_ps` and `mm_hadd_ps` where both significantly slower even though it results in fewer instructions. I suspect that the issue is that for a single dot product the overhead of loading in and out of mm128 registers is more cost than it's worth.
Naive Rust version output
.cfi_startproc
pushq %rbp
.cfi_def_cfa_offset 16
.cfi_offset %rbp, -16
movq %rsp, %rbp
.cfi_def_cfa_register %rbp
vmovss (%rdi), %xmm0
vmulss (%rsi), %xmm0, %xmm0
vmovsd 4(%rdi), %xmm1
vmovsd 4(%rsi), %xmm2
vmulps %xmm2, %xmm1, %xmm1
vaddss %xmm1, %xmm0, %xmm0
vmovshdup %xmm1, %xmm1
vaddss %xmm1, %xmm0, %xmm0
popq %rbp
retq
.cfi_startproc
pushq %rbp
.cfi_def_cfa_offset 16
.cfi_offset %rbp, -16
movq %rsp, %rbp
.cfi_def_cfa_register %rbp
vmovaps (%rdi), %xmm0
vmulps (%rsi), %xmm0, %xmm0
vhaddps %xmm0, %xmm0, %xmm0
vhaddps %xmm0, %xmm0, %xmm0
popq %rbp
retq
0: https://github.com/k0nserv/rusttracer/commits/SIMD-mm256-dp-...
Re: Revisiting the Business Card Raytracer
#35Nice work on the GPU programming, and the multicore before that, but I'm mystified why going from -O0 to -O3 is named an "optimisation". All respect for Fabien, but running code that's supposed to run faster than a snail (and if you're not debugging and require -O0 for reasonable output) implies -O2 or -O3. (In practice, -O3 often doesn't give much performance over -O2, despite increasing compile times.) The initial…
Agree with you
Me too
Re: Revisiting the Business Card Raytracer
#36Earlier quoted context omitted.
Off topic: I teach compilers in high school and godbolt.org looks amazing, thanks for the link!
Wow, really cool that there are high schools teaching compilers.
Yes, in Portuguese high schools you can do a technical education during the last three years (10 - 12), that gets you ready for the job market.
I did mine with focus on informatics during in the late 80's/early 90's.
Brief overview of three years subjects, besides the usual high school stuff.
Graphics programming, compilers, databases, MS-DOS, UNIX (Xenix back then), Networking (Novell Netware), OS development.
Languages that we got to use for different kinds of assignments during those three years, GW-Basic, Turbo Basic, Turbo Pascal 5.5, Turbo C 2.0/K&R C, Turbo C++ 1.0, Dbase III+, Clipper Summer '87 and OOP variant Clipper 5.x, 8086 and 68000 Assembly.
The high school I took it on still offers this, naturally updated to more modern stacks and teaching subjects.
Re: Revisiting the Business Card Raytracer
#37Earlier quoted context omitted.
Looking at it on Godbolt, it doesn't really leverage SSE on -O3, either. You can get a reasonable grasp of whether it's using SSE effectively or not just by looking at the instruction names. mulss: multiplication of a single single-precision floating point value. mulsd: multiplication of a single double-precision floating point value. mulps: multiplication of a packed group of single-precision floating point values.…
This post is really topical for me. I spent hours yesterday trying to write explicit SIMD code[0] for my the vector dot product in my raytracer and all I managed to do was slow the code down about 20-30%. The code generated by Rust from the naive solution uses ss instructions mostly whereas my two tries using `mm_dp_ps` and `mm_mul_ps` and `mm_hadd_ps` where both significantly slower even though it results in fewer i…
I'm actually rather bad at this, but my understanding is that the horizontal operations are relatively slow. The easiest way to get throughput out of SIMD is to have a structure representing 4 points, with a vec4 of your X values, a vec4 of your Y values, and a vec4 of your Z values. Then you can do 4 dot products easily and efficiently, using only a handful of vertical packed instructions. (If figuring out how to effectively use a structure like that sounds difficult and annoying, that's because it is.)
Re: Revisiting the Business Card Raytracer
#38A Ray-Tracer in 7 Lines of K: http://nsl.com/k/ray/ray.k
Re: Revisiting the Business Card Raytracer
#39Earlier quoted context omitted.
This post is really topical for me. I spent hours yesterday trying to write explicit SIMD code[0] for my the vector dot product in my raytracer and all I managed to do was slow the code down about 20-30%. The code generated by Rust from the naive solution uses ss instructions mostly whereas my two tries using `mm_dp_ps` and `mm_mul_ps` and `mm_hadd_ps` where both significantly slower even though it results in fewer i…
I'm actually rather bad at this, but my understanding is that the horizontal operations are relatively slow. The easiest way to get throughput out of SIMD is to have a structure representing 4 points, with a vec4 of your X values, a vec4 of your Y values, and a vec4 of your Z values. Then you can do 4 dot products easily and efficiently, using only a handful of vertical packed instructions. (If figuring out how to ef…
Yeah that was my conclusion too. I don't think I have any cases where the need to perform the dot product between multuple paris of vectors arise however, at least not anywhere in the hot loop where it would help. Raytracers tend to use a lot of dot products followed by some checks then another dot product but there's a strictly sequential process in these algorithms.
Re: Revisiting the Business Card Raytracer
#40Earlier quoted context omitted.
Wow, really cool that there are high schools teaching compilers.
Yes, in Portuguese high schools you can do a technical education during the last three years (10 - 12), that gets you ready for the job market. I did mine with focus on informatics during in the late 80's/early 90's. Brief overview of three years subjects, besides the usual high school stuff. Graphics programming, compilers, databases, MS-DOS, UNIX (Xenix back then), Networking (Novell Netware), OS development. Langu…
For comparison, my Canadian high school offered a "Teach Yourself C++ in 30 Days" book that you could study for up to 10 hours in the optional Computers course. If you chose that module, by the end of the first class, you would be more knowledgeable on the subject than any teacher in the school.
(It was actually an excellent school; they just did not care about computing. Nevertheless, I'm quite jealous of those kids with such an interesting option available to them.)