syphilis, I see the same problem with strawberry perl 5.24.1 for MSWin32-x64 with gcc 4.9.2, so I added a function in the c to dump the bytes (I didn't want the bytes leaving the c-environment, just in case trying to follow BrowserUK's unpack advice introduced a translation error (as he said, "might change things"):

//////////////////////////////////////////////////////////////////////
+//////////////////////////////////////////////////////////
// https://perlmonks.org/?node_id=1189780
//////////////////////////////////////////////////////////////////////
+//////////////////////////////////////////////////////////
#include <stdio.h>
#include <stdlib.h>
#include <string.h>

void _dump64(char* varname, _Decimal64 d64) {
    int i;
    void*   ptr = (void*)(&d64);
    printf("%s = 0x", varname);
    for(i=0; i < sizeof(d64); i++) {
        printf("%02X ", 0xFF & *(char*)(ptr+i));
    }
    printf("\n");
}
#define dump64(v) _dump64(#v, v)

int foo(void) {
    _Decimal64 pz64 =  0.DD, nz64 = 0.DD, no64 = -1.DD;
    nz64 = pz64 * no64;
    dump64(pz64);
    dump64(nz64);
    dump64(no64);
    if(nz64 !=  0.DD) printf("!= +0\n"); else printf("== +0\n");
    if(nz64 != -0.DD) printf("!= -0\n"); else printf("== -0\n");
    return 0;
}

int main(int argc, char* argv) {
    foo();
}

-----------------------------------------
pz64 = 0x00 00 00 00 00 00 C0 31
nz64 = 0x00 00 00 00 00 00 C0 B1
no64 = 0x01 00 00 00 00 00 C0 B1
== +0
== -0

[download]

When I converted that to perl, and run using Inline::C 0.76, I get

pz64 = 0x00 00 00 00 00 00 C0 31
nz64 = 0x00 00 00 00 00 00 C0 B1
no64 = 0x01 00 00 00 00 00 C0 B1
!= +0
== -0

[download]

That's definitely a strange one: inside C, the bits are the same, but the comparison-behavior is different.

I see a lot of compiler options during the make- and make-install stages, and I'm wondering if one of them is messing up (intentionally modifying?) gcc behavior. What I would probably do next (if it were a problem I had to solve -- or if I had more time to spend on it today for my own education, but unfortunately, $work beckons for now): start making your pure-c compile look more like the Inline::C compile, adding more of the options, until such time as the pure-c failed... 

  Starting "make" Stage
Running Mkbootstrap for pm1189780_pl_3d28 ()
"C:\usr\local\apps\strawberry\perl\bin\perl.exe" -MExtUtils::Command -
+e chmod -- 644 "pm1189780_pl_3d28.bs"
"C:\usr\local\apps\strawberry\perl\bin\perl.exe" -MExtUtils::Command::
+MM -e cp_nonempty -- pm1189780_pl_3d28.bs blib\arch\auto\pm1189780_pl
+_3d28\pm1189780_pl_3d28.bs 644
"C:\usr\local\apps\strawberry\perl\bin\perl.exe" "C:\usr\local\apps\st
+rawberry\perl\lib\ExtUtils\xsubpp"  -typemap "C:\usr\local\apps\straw
+berry\perl\lib\ExtUtils\typemap"  pm1189780_pl_3d28.xs > pm1189780_pl
+_3d28.xsc
"C:\usr\local\apps\strawberry\perl\bin\perl.exe" -MExtUtils::Command -
+e mv -- pm1189780_pl_3d28.xsc pm1189780_pl_3d28.c
gcc -c  -I"C:/usr/local/share/passthru/perl/perlmonks"  -s -O2 -DWIN32
+ -DWIN64 -DCONSERVATIVE -DPERL_TEXTMODE_SCRIPTS -DPERL_IMPLICIT_CONTE
+XT -DPERL_IMPLICIT_SYS -fwrapv -fno-strict-aliasing -mms-bitfields -s
+ -O2     -DVERSION=\"0.00\"    -DXS_VERSION=\"0.00\"  "-IC:\usr\local
+\apps\strawberry\perl\lib\CORE"   pm1189780_pl_3d28.c
"C:\usr\local\apps\strawberry\perl\bin\perl.exe" -MExtUtils::Mksymlist
+s \
     -e "Mksymlists('NAME'=>\"pm1189780_pl_3d28\", 'DLBASE' => 'pm1189
+780_pl_3d28', 'DL_FUNCS' => {  }, 'FUNCLIST' => [], 'IMPORTS' => {  }
+, 'DL_VARS' => []);"
g++.exe pm1189780_pl_3d28.def -o blib\arch\auto\pm1189780_pl_3d28\pm11
+89780_pl_3d28.xs.dll -mdll -s -L"C:\usr\local\apps\strawberry\perl\li
+b\CORE" -L"C:\usr\local\apps\strawberry\c\lib" pm1189780_pl_3d28.o   
+"C:\usr\local\apps\strawberry\perl\lib\CORE\libperl524.a" -lmoldname 
+-lkernel32 -luser32 -lgdi32 -lwinspool -lcomdlg32 -ladvapi32 -lshell3
+2 -lole32 -loleaut32 -lnetapi32 -luuid -lws2_32 -lmpr -lwinmm -lversi
+on -lodbc32 -lodbccp32 -lcomctl32 -Wl,--enable-auto-image-base
"C:\usr\local\apps\strawberry\perl\bin\perl.exe" -MExtUtils::Command -
+e chmod -- 755 blib\arch\auto\pm1189780_pl_3d28\pm1189780_pl_3d28.xs.
+dll
  Finished "make" Stage

  Starting "make install" Stage
"C:\usr\local\apps\strawberry\perl\bin\perl.exe" -MExtUtils::Command::
+MM -e cp_nonempty -- pm1189780_pl_3d28.bs blib\arch\auto\pm1189780_pl
+_3d28\pm1189780_pl_3d28.bs 644
Files found in blib\arch: installing files in blib\lib into architectu
+re dependent library tree
Installing C:\usr\local\share\passthru\perl\perlmonks\_Inline\lib\auto
+\pm1189780_pl_3d28\pm1189780_pl_3d28.xs.dll
  Finished "make install" Stage

[download]

Let us know if you find anything more...

In reply to Re^2: Inline::C producing an absurd result by pryrt
in thread Inline::C producing an absurd result by syphilis

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