exp implementation

Christopher Howard

11 Apr 2013 11 Apr '13

1:38 a.m.

Hi. For my own learning, I wanted to see how the exp function is implemented in GHC. I have GHC 7.4.1 source code open, but I'm having trouble figuring out which file the actual function definition is in. I see expFloat (F# x) = F# (expFloat# x) in libraries/base/GHC/Float.lhs. -- frigidcode.com

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Brandon Allbery

11 Apr 11 Apr

10:37 a.m.

On Thu, Apr 11, 2013 at 1:38 AM, Christopher Howard < christopher.howard@frigidcode.com> wrote:

...

Hi. For my own learning, I wanted to see how the exp function is implemented in GHC. I have GHC 7.4.1 source code open, but I'm having trouble figuring out which file the actual function definition is in. I see

expFloat (F# x) = F# (expFloat# x)

expFloat# is likely a primop; good luck.... Primops aka primitive operations are generally implemented in the compiler backend as assembly language or Cmm code. Untangling that part of ghc makes my head swim. >.> -- brandon s allbery kf8nh sine nomine associates allbery.b@gmail.com ballbery@sinenomine.net unix, openafs, kerberos, infrastructure, xmonad http://sinenomine.net

Christopher Howard

11:12 a.m.

On 04/11/2013 06:37 AM, Brandon Allbery wrote:

...

On Thu, Apr 11, 2013 at 1:38 AM, Christopher Howard mailto:christopher.howard@frigidcode.com> wrote:

Hi. For my own learning, I wanted to see how the exp function is implemented in GHC. I have GHC 7.4.1 source code open, but I'm having trouble figuring out which file the actual function definition is in. I see

expFloat (F# x) = F# (expFloat# x)

expFloat# is likely a primop; good luck.... Primops aka primitive operations are generally implemented in the compiler backend as assembly language or Cmm code. Untangling that part of ghc makes my head swim. >.>

-- brandon s allbery kf8nh sine nomine associates allbery.b@gmail.com mailto:allbery.b@gmail.com ballbery@sinenomine.net mailto:ballbery@sinenomine.net unix, openafs, kerberos, infrastructure, xmonad http://sinenomine.net

I traced it down some more: I think it eventually goes into the compiler/nativeGen section where it is translated into the platform's native version of the function. On my platform, I think this is the expf function from math.h. (See EXP(3)). I find that to be interesting, because it means you could change the output of your programs by altering your standard library. But I guess there are a lot of things you could change by altering your standard library! -- frigidcode.com

Christopher Howard

11:30 a.m.

On 04/11/2013 07:12 AM, Christopher Howard wrote:

...

On 04/11/2013 06:37 AM, Brandon Allbery wrote:

I traced it down some more: I think it eventually goes into the compiler/nativeGen section where it is translated into the platform's native version of the function. On my platform, I think this is the expf function from math.h. (See EXP(3)).

I find that to be interesting, because it means you could change the output of your programs by altering your standard library. But I guess there are a lot of things you could change by altering your standard library!

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In glibc, it eventual comes down to this approximation, I think (from ./sysdeps/ieee754/flt-32/e_expf.c): code: -------- /* Compute ex2 = 2^n e^(t/512+delta[t]). */ ex2_u.d = __exp_atable[tval+177]; ex2_u.ieee.exponent += (int) n; /* Approximate e^(dx+delta) - 1, using a second-degree polynomial, with maximum error in [-2^-10-2^-28,2^-10+2^-28] less than 5e-11. */ x22 = (0.5000000496709180453 * dx + 1.0000001192102037084) * dx + delta; /* ... snip ... */ result = x22 * ex2_u.d + ex2_u.d; -------- -- frigidcode.com

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