From:

Date:

February 16, 2003 04:04Subject:

Re: [perl #20827] Unexpected scientific notation.Message ID:

200302161206.h1GC6a727430@crypt.compulink.co.ukhv@crypt.org wrote: :Nicholas Clark <nick@unfortu.net> wrote: ::If anything, the peephole optimiser should be changed, and the constant ::1597009560 ** 2 left as an NV. This would make the first two the same, and ::the second two both scientific notation. : :I agree they should be the same. I think the answer though is to move the :optimisation (hmm, correction) to where it belongs. [...] :The simplistic patch below breaks no tests here. I've applied a tweaked version (below) of that patch as change #18720. This also removes the original compile-time attempts to force to integer. :And fifthly, should be returning (IV)1 when power is (IV)0, regardless :of the type (assuming non-zero) of the base? I've left this one out for now. Hugo --- op.c.old Sun Feb 16 12:55:13 2003 +++ op.c Sun Feb 16 12:33:32 2003 @@ -1942,19 +1942,7 @@ op_free(o); if (type == OP_RV2GV) return newGVOP(OP_GV, 0, (GV*)sv); - else { - /* try to smush double to int, but don't smush -2.0 to -2 */ - if ((SvFLAGS(sv) & (SVf_IOK|SVf_NOK|SVf_POK)) == SVf_NOK && - type != OP_NEGATE) - { -#ifdef PERL_PRESERVE_IVUV - /* Only bother to attempt to fold to IV if - most operators will benefit */ - SvIV_please(sv); -#endif - } - return newSVOP(OP_CONST, 0, sv); - } + return newSVOP(OP_CONST, 0, sv); nope: return o; --- pp.c.old Sun Feb 16 12:55:39 2003 +++ pp.c Sun Feb 16 12:55:24 2003 @@ -879,16 +879,15 @@ PP(pp_pow) { - dSP; dATARGET; tryAMAGICbin(pow,opASSIGN); + dSP; dATARGET; #ifdef PERL_PRESERVE_IVUV - /* ** is implemented with pow. pow is floating point. Perl programmers - write 2 ** 31 and expect it to be 2147483648 - pow never made any guarantee to deliver a result to 53 (or whatever) - bits of accuracy. Which is unfortunate, as perl programmers expect it - to, and on some platforms (eg Irix with long doubles) it doesn't in - a very visible case. (2 ** 31, which a regression test uses) - So we'll implement power-of-2 ** +ve integer with multiplies, to avoid - these problems. */ + bool is_int = 0; +#endif + tryAMAGICbin(pow,opASSIGN); +#ifdef PERL_PRESERVE_IVUV + /* For integer to integer power, we do the calculation by hand wherever + we're sure it is safe; otherwise we call pow() and try to convert to + integer afterwards. */ { SvIV_please(TOPm1s); if (SvIOK(TOPm1s)) { @@ -920,10 +919,12 @@ goto float_it; /* Can't do negative powers this way. */ } } - /* now we have integer ** positive integer. - foo & (foo - 1) is zero only for a power of 2. */ + /* now we have integer ** positive integer. */ + is_int = 1; + + /* foo & (foo - 1) is zero only for a power of 2. */ if (!(baseuv & (baseuv - 1))) { - /* We are raising power-of-2 to postive integer. + /* We are raising power-of-2 to a positive integer. The logic here will work for any base (even non-integer bases) but it can be less accurate than pow (base,power) or exp (power * log (base)) when the @@ -935,20 +936,6 @@ NV base = baseuok ? baseuv : -(NV)baseuv; int n = 0; - /* The logic is this. - x ** n === x ** m1 * x ** m2 where n = m1 + m2 - so as 42 is 32 + 8 + 2 - x ** 42 can be written as - x ** 32 * x ** 8 * x ** 2 - I can calculate x ** 2, x ** 4, x ** 8 etc trivially: - x ** 2n is x ** n * x ** n - So I loop round, squaring x each time - (x, x ** 2, x ** 4, x ** 8) and multiply the result - by the x-value whenever that bit is set in the power. - To finish as soon as possible I zero bits in the power - when I've done them, so that power becomes zero when - I clear the last bit (no more to do), and the loop - terminates. */ for (; power; base *= base, n++) { /* Do I look like I trust gcc with long longs here? Do I hell. */ @@ -956,24 +943,69 @@ if (power & bit) { result *= base; /* Only bother to clear the bit if it is set. */ - power &= ~bit; + power -= bit; /* Avoid squaring base again if we're done. */ if (power == 0) break; } } SP--; SETn( result ); + SvIV_please(TOPs); RETURN; - } - } - } + } else { + register unsigned int highbit = 8 * sizeof(UV); + register unsigned int lowbit = 0; + register unsigned int diff; + while ((diff = (highbit - lowbit) >> 1)) { + if (baseuv & ~((1 << (lowbit + diff)) - 1)) + lowbit += diff; + else + highbit -= diff; + } + /* we now have baseuv < 2 ** highbit */ + if (power * highbit <= 8 * sizeof(UV)) { + /* result will definitely fit in UV, so use UV math + on same algorithm as above */ + register UV result = 1; + register UV base = baseuv; + register int n = 0; + for (; power; base *= base, n++) { + register UV bit = (UV)1 << (UV)n; + if (power & bit) { + result *= base; + power -= bit; + if (power == 0) break; + } + } + SP--; + if (baseuok || !(power & 1)) + /* answer is positive */ + SETu( result ); + else if (result <= (UV)IV_MAX) + /* answer negative, fits in IV */ + SETi( -(IV)result ); + else if (result == (UV)IV_MIN) + /* 2's complement assumption: special case IV_MIN */ + SETi( IV_MIN ); + else + /* answer negative, doesn't fit */ + SETn( -(NV)result ); + RETURN; + } + } + } + } } - float_it: + float_it: #endif { - dPOPTOPnnrl; - SETn( Perl_pow( left, right) ); - RETURN; + dPOPTOPnnrl; + SETn( Perl_pow( left, right) ); +#ifdef PERL_PRESERVE_IVUV + if (is_int) + SvIV_please(TOPs); +#endif + RETURN; } }Thread Previous | Thread Next

- Re: [perl #20827] Unexpected scientific notation. by Abigail
- Re: [perl #20827] Unexpected scientific notation. by John Peacock
- Re: [perl #20827] Unexpected scientific notation. by Nicholas Clark
- [perl #20827] Unexpected scientific notation. by perlbug-followup
- Re: [perl #20827] Unexpected scientific notation. by Nicholas Clark
- Re: [perl #20827] Unexpected scientific notation. by hv
**Re: [perl #20827] Unexpected scientific notation.**by hv- Re: [perl #20827] Unexpected scientific notation. by Nicholas Clark
- Re: [perl #20827] Unexpected scientific notation. by John Peacock

nntp.perl.org: Perl Programming lists via nntp and http.

Comments to Ask Bjørn Hansen at ask@perl.org | Group listing | About