[llvm-commits] [PATCH][FastMath, InstCombine] Fadd/Fsub optimizations

[Stephen Canon]

(x + x) + x --> x*3 is always exact and does not require relaxed / fast-math.

- Steve

On Dec 11, 2012, at 4:32 PM, Shuxin Yang <shuxin.llvm at gmail.com> wrote:

> Hi, Dear All:
> 
>  The attached patch is to implement following rules about floating-point add/sub in relaxed mode.
> (The n-th rule is not yet implemented. I just realized it when I write this mail.
> It is easy to implement this rule, but I don't like to go through stress test one more time).
> 
> ----------------------------------------------------
> 1. (x + c1) + c2 ->  x + (c1 + c2)
> 2. (c * x) + x -> (c+1) * x
> 3. (x + x) + x -> x * 3
> 4. c * x + (x + x) -> (c + 2)*x
> 5. (x + x) + (x+x) -> 4*x
> 6. x - (x + y) -> 0 - y
>  ...
>  ...
>  ...
> n. (factoring) C * X1 + C * X2 -> C(X1 + X2)
> -------------------------------------------------------
> 
>  Up to three neighboring instructions are involved in the optimization. The number
> of the combination is daunting!. So I have to resort a general way (instead of
> pattern match) to tackle these optimizations.
> 
>  The idea is simple, just try to decompose instructions into uniformally represented
> Addends. Take following instruction sequence as an example:
> 
>  t1 = 1.8 * x;
>  t2 = y - x;
>  t3 = t1 - t2;
> 
> t3 has two addends A1=<1, t1> (denote value 1*t1), and A2=<-1, t2>. If we "zoom-in"
> A1 and A2 one step, we will reveal more addends: A1 can be zoom-in-ed into another
> addend A1_0 = <1.8, x>, and A2 can be zoom-in into <1,y> and <-1,x>.
> 
> When these addends available, the optimize try to optimize following N-ary additions
> using symbolic evaluation:
>   A1_0 + A2_0 + A2_1, or
>   A1 +  A2_0 + A2_1 or
>   A1_0 + A2
> 
> This patch is stress-tested with SingleSrc and MultiSource by considering all fadd/fsub
> are in relaxed mode.
> 
> Thank you for code review!
> 
> Shuxin
> 
> <fast_math.add_sub.v1.patch>_______________________________________________
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