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Re^2: Inline::C producing an absurd result

by syphilis (Chancellor)
on May 08, 2017 at 13:19 UTC ( #1189795=note: print w/replies, xml ) Need Help??


in reply to Re: Inline::C producing an absurd result
in thread Inline::C producing an absurd result

My only suggestion is that you dump the values as bits and see where they differ

Definitely worth a try, but it appears not to have helped much.
So .... I changed the original Inline::C script to:
use strict; use warnings; use Inline C => Config => BUILD_NOISY => 1, USING => 'ParseRegExp', #CLEAN_AFTER_BUILD => 0, ; use Inline C => <<'EOC'; #include <stdio.h> int foo(void) { _Decimal64 d64 = 0.DD, neg_zero, pos_zero = 0.DD; void *pd64 = &d64, *pneg_zero = &neg_zero, *ppos_zero = &pos_zero; int i; neg_zero = pos_zero * -1.DD; d64 *= -1.DD; if(d64 != pos_zero) printf("!= 0\n"); else printf("== 0\n"); if(d64 != neg_zero) printf("!= -0\n"); else printf("== -0\n"); for(i = 7; i >= 0; i--) printf("%02X", ((unsigned char*)pd64)[i]); printf("\n"); for(i = 7; i >= 0; i--) printf("%02X", ((unsigned char*)pneg_zero)[i]); printf("\n"); for(i = 7; i >= 0; i--) printf("%02X", ((unsigned char*)ppos_zero)[i]); printf("\n"); return 0; } EOC foo();
It now outputs:
!= 0 == -0 B1C0000000000000 B1C0000000000000 31C0000000000000
"B1C000..." is a _Decimal64 -0, and "31C000..." a _Decimal64 0.
The first bit is the sign bit, the next 10 bits encode the exponent, and the 54-bit significand is an implicit 0 bit followed by the remaining 53 bits (all 0).
When I run that revised C code as a C program it outputs (correctly):
== 0 == -0 B1C0000000000000 B1C0000000000000 31C0000000000000
I also modified the Inline::C rendition that was getting it right to:
use strict; use warnings; use Inline C => Config => BUILD_NOISY => 1, USING => 'ParseRegExp', #CLEAN_AFTER_BUILD => 0, ; use Inline C => <<'EOC'; #include <stdio.h> void _is_zero(_Decimal64 d64) { _Decimal64 neg_zero, pos_zero = 0.DD; void *pd64 = &d64, *pneg_zero = &neg_zero, *ppos_zero = &pos_zero; int i; neg_zero = pos_zero * -1.DD; if(d64 != pos_zero) printf("!= 0\n"); else printf("== 0\n"); if(d64 != neg_zero) printf("!= -0\n"); else printf("== -0\n"); for(i = 7; i >= 0; i--) printf("%02X", ((unsigned char*)pd64)[i]); printf("\n"); for(i = 7; i >= 0; i--) printf("%02X", ((unsigned char*)pneg_zero)[i]); printf("\n"); for(i = 7; i >= 0; i--) printf("%02X", ((unsigned char*)ppos_zero)[i]); printf("\n"); } int foo(void) { _Decimal64 d64 = 0.DD; d64 *= -1.DD; _is_zero(d64); return 0; } EOC foo();
It still outputs (correctly):
== 0 == -0 B1C0000000000000 B1C0000000000000 31C0000000000000
It's as though the != operation (in the problem script) believes that the values it's comparing are unsigned.

Cheers,
Rob

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