Questions tagged [fma]
Fused Multiply Add or Multiply-Accumulate
73
questions
47votes
2answers
36kviews
How to use Fused Multiply-Add (FMA) instructions with SSE/AVX
I have learned that some Intel/AMD CPUs can do simultanous multiply and add with SSE/AVX: FLOPS per cycle for sandy-bridge and haswell SSE2/AVX/AVX2.
I like to know how to do this best in code and I ...
user2088790
47votes
1answer
8kviews
Obtaining peak bandwidth on Haswell in the L1 cache: only getting 62%
I'm attempting to obtain full bandwidth in the L1 cache for the following function on Intel processors
float triad(float *x, float *y, float *z, const int n) {
float k = 3.14159f;
for(int i=0;...
- 30.9k
36votes
2answers
3kviews
Significant FMA performance anomaly experienced in the Intel Broadwell processor
Code1:
vzeroall
mov rcx, 1000000
startLabel1:
vfmadd231ps ymm0, ymm0, ymm0
vfmadd231ps ymm1, ymm1, ymm1
vfmadd231ps ymm2, ymm2, ymm2
vfmadd231ps ymm3, ymm3, ymm3
...
- 549
24votes
2answers
16kviews
FMA3 in GCC: how to enable
I have a i5-4250U which has AVX2 and FMA3. I am testing some dense matrix multiplication code in GCC 4.8.1 on Linux which I wrote. Below is a list of three difference ways I compile.
SSE2: gcc ...
- 30.9k
18votes
1answer
5kviews
AVX2: Computing dot product of 512 float arrays
I will preface this by saying that I am a complete beginner at SIMD intrinsics.
Essentially, I have a CPU which supports the AVX2 instrinsic (Intel(R) Core(TM) i5-7500T CPU @ 2.70GHz). I would like ...
- 919
16votes
6answers
4kviews
Which algorithms benefit most from fused multiply add?
fma(a,b,c) is equivalent to a*b+c except it doesn't round intermediate result.
Could you give me some examples of algorithms that non-trivially benefit from avoiding this rounding?
It's not obvious, ...
- 17.3k
15votes
4answers
10kviews
How to get data out of AVX registers?
Using MSVC 2013 and AVX 1, I've got 8 floats in a register:
__m256 foo = mm256_fmadd_ps(a,b,c);
Now I want to call inline void print(float) {...} for all 8 floats. It looks like the Intel AVX ...
- 166k
15votes
2answers
2kviews
Fused multiply add and default rounding modes
With GCC 5.3 the following code compield with -O3 -fma
float mul_add(float a, float b, float c) {
return a*b + c;
}
produces the following assembly
vfmadd132ss %xmm1, %xmm2, %xmm0
ret
I ...
- 30.9k
14votes
3answers
2kviews
Can I use the AVX FMA units to do bit-exact 52 bit integer multiplications?
AXV2 doesn't have any integer multiplications with sources larger than 32-bit. It does offer 32 x 32 -> 32 multiplies, as well as 32 x 32 -> 64 multiplies1, but nothing with 64-bit sources.
Let's say ...
- 55.9k
14votes
2answers
6kviews
Why does the FMA _mm256_fmadd_pd() intrinsic have 3 asm mnemonics, "vfmadd132pd", "231" and "213"?
Could someone explain to me why there are 3 variants of the fused multiply-accumulate instruction: vfmadd132pd, vfmadd231pd and vfmadd213pd, while there is only one C intrinsics _mm256_fmadd_pd?
To ...
- 61.7k
10votes
1answer
796views
Do FMA (fused multiply-add) instructions always produce the same result as a mul then add instruction?
I have this assembly (AT&T syntax):
mulsd %xmm0, %xmm1
addsd %xmm1, %xmm2
I want to replace it with:
vfmadd231sd %xmm0, %xmm1, %xmm2
Will this transformation always leave equivalent state ...
- 3,144
10votes
3answers
1kviews
Optimize for fast multiplication but slow addition: FMA and doubledouble
When I first got a Haswell processor I tried implementing FMA to determine the Mandelbrot set. The main algorithm is this:
intn = 0;
for(int32_t i=0; i<maxiter; i++) {
floatn x2 = square(x), ...
- 30.9k
9votes
2answers
10kviews
Preventing GCC from automatically using AVX and FMA instructions when compiled with -mavx and -mfma
How can I disable auto-vectorization with AVX and FMA instructions? I would still prefer the compiler to employ SSE and SSE2 automatically, but not FMA and AVX.
My code that uses AVX checks for its ...
- 30.4k
9votes
2answers
3kviews
Automatically generate FMA instructions in MSVC
MSVC supports AVX/AVX2 instructions for years now and according to this msdn blog post, it can automatically generate fused-multiply-add (FMA) instructions.
Yet neither of the following functions ...
- 7,698
8votes
2answers
11kviews
How do I know if I can compile with FMA instruction sets?
I have seen questions about how to use FMA instructions set but before I get to start using them, I'd first like to know if I can (does my processor support them). I found a post saying that I needed ...
- 3,211