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.
