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Correction for function-math-fma.md

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Fixed `VFMADD231SD` to be `VFMADD213SD` as the operation order for the functions differ
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Wunder Wulfe 2025-02-23 01:02:01 -03:00 • committed by GitHub
parent edc2949a26
commit d213b82f91
Signed by: DevComp
GPG key ID: B5690EEEBB952194
1 changed files with 4 additions and 4 deletions
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8
docs/function-math-fma.md
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@ -61,9 +61,9 @@ The *two* advantages of this operation are as follows:
The optimization will reduce to `4` math instructions:
```asm
MULSD xmm3, xmm7 ; aw * bw
VFMADD231SD xmm2, xmm6, xmm3 ; az * bz + (aw * bw)
VFMADD231SD xmm1, xmm5, xmm2 ; ay * by + (az * bz + aw * bw)
VFMADD231SD xmm0, xmm4, xmm1 ; ax * bx + (ay * by + az * bz + aw * bw)
VFMADD213SD xmm2, xmm6, xmm3 ; az * bz + (aw * bw)
VFMADD213SD xmm1, xmm5, xmm2 ; ay * by + (az * bz + aw * bw)
VFMADD213SD xmm0, xmm4, xmm1 ; ax * bx + (ay * by + az * bz + aw * bw)
```
There is also the potential of updating libraries such as `vector` to make use of `math.fma` internally, and other cases such as Roblox's `CFrame` could also benefit from this change.
@ -91,7 +91,7 @@ When generating `native` code with `--!native` or `@native`, the operation shoul
The compiler would attempt to optimize the interior of `sqrt(x * x - y * y)` into `(x * x) - (y * y)` and eventually `fma(x, x, -(y * y))`, which should compile to the following arithmetic instructions:
```asm
MULSD xmm1, xmm1 ; y * y
VFMSUB231SD xmm0, xmm0, xmm1 ; x * x - (y * y)
VFMSUB213SD xmm0, xmm0, xmm1 ; x * x - (y * y)
```
This is problematic, as there is a gap between the precision of the operations. `a * b` might produce a rounding error which is different from `a * b - c`, and a negative value could unintentionally be introduced to the square root, even if `x == y`, resulting in an error which previously would not have existed.