dmd 2.029 release
Don
nospam at nospam.com
Thu Apr 23 00:59:39 PDT 2009
bearophile wrote:
> Don:
>> Actually it's not so difficult. I've created a patch for bug 2807 --
>> it's only 5 lines long! It gives an error message if a nested pure
>> function accesses a mutable variable from an outer scope.
>
> Thank you very much Don, your work helps a lot.
>
> Every time I try a tiny program I find something I don't understand:
>
> import std.stdio: writeln;
> import std.math: sqrt;
> import std.conv: to;
>
> void main(string[] args) {
> double x = args.length == 2 ? to!(double)(args[1]) : 4.0;
> writeln(sqrt(x) + sqrt(x));
> }
>
> I have also tried with std.math.sin with similar results:
>
> L0: enter 8,0
> mov EAX,8[EBP]
> cmp EAX,2
> jne L30
> cmp EAX,1
> ja L1B
> mov EAX,6
> call near ptr _D6test7__arrayZ
> L1B: mov EDX,0Ch[EBP]
> mov EAX,8[EBP]
> mov EAX,8[EDX]
> mov EDX,0Ch[EDX]
> push EDX
> push EAX
> call near ptr _D3std4conv13__T2toTdTAyaZ2toFAyaZd
> jmp short L36
> L30: fld qword ptr FLAT:_DATA[00h]
> L36: fstp qword ptr -8[EBP]
> fld qword ptr -8[EBP]
> fsin
> fld qword ptr -8[EBP]
> fsin
> faddp ST(1),ST
> sub ESP,0Ch
> fstp tbyte ptr [ESP]
> call near ptr _D3std5stdio14__T7writelnTeZ7writelnFeZv
> xor EAX,EAX
> leave
> ret
>
> Isn't sin(x)+sin(x) pure? Even if the compiler doesn't want to replace x+x with x*2 because x is a floating point, it can do:
>
> y = sin(x)
> y+y
>
> And that gives the same result even with FPs.
Yes. From further investigation, I've found that:
(1) the optimisation of pure only happens for int returns, not for
floating-point return types. It should convert purefunc(x)+purefunc(x)
into 2*purefunc(x) if the return type of purefunc is int, float, or complex.
(2) the back-end isn't smart enough to convert f*2 into f+f.
It's difficult to work out where it's optimising the int return. I can
see where it marks pure calls as subexpressions to be potentially
eliminated: OTae(op) is true in gflow.c(660) if it's a pure call, false
if it's an impure call.
The problem may simply be that the backend doesn't do much common
subexpression elimination of floating-point expressions.
I really don't understand the backend. It's quite cryptic. Key acronyms
are AE, CP and VBE. Then there's Bin, Bgen, Bkill, etc.
AE *might* be Available Expression (but what does that mean?)
CP might be Copy Propagation info
I've found that VBE = "Very Busy Expression"! (what does that mean?)
Fixing your problem is beyond me at present, I think.
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