c - floating point multiplication using bit manipulation -


this question has answer here:

i'm trying solve problem need return bit-level equivalent of unsigned floating point number times 4.

so far i've been exploring other answers , found 1 pretty similar mine, didn't work. there wasn't explanation given i'm trying understand getting wrong when run test cases.

when test problems

33554432[0x2000000].

but should

25165824[0x1800000]

so far code is

unsigned = (uf >> 0x17) & 0xff; unsigned b = uf & 0x80000000; unsigned c = uf & 0x007fffff; if (a == 0xff || (a == 0x0 && c == 0x0))       return uf; if (a) {     <<= 0x2; } else if (c == 0x7fffff) {     c >>= 0x2;     <<= 0x2; } else {     c <<= 0x2; }  return (a<<0x17|b|c);

any advice?

unless writing software floating-point emulator (in case need more can give you), should let compiler handle this:

#include <stdint.h> #include <assert.h>  static_assert(sizeof(float) == sizeof(uint32_t));  uint32_t multiply_by_4_as_float(uint32_t n) {     union {         uint32_t u;         float f;     } conv;      conv.u = n;     conv.f *= 4;     return conv.u; }

(this use of unions has unspecified, not undefined, behavior in c1999 technical corrigienda applied, , in c2011. in c1989 undefined, , in c++ may still undefined.)

(if you're asking question because need little bit of floating-point math in program can't use hardware floating point unit concrete reason, check compiler's documentation - there may option can set makes use own software fp. keep in mind software floating point orders of magnitude slower hardware.)


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