This is the mail archive of the mailing list for the GCC project.

Index Nav: [Date Index] [Subject Index] [Author Index] [Thread Index]
Message Nav: [Date Prev] [Date Next] [Thread Prev] [Thread Next]
Other format: [Raw text]

Re: Option to make unsigned->signed conversion always well-defined?

On Thu, Oct 6, 2011 at 11:31 PM, Florian Weimer <> wrote:
> * Ulf Magnusson:
>> I've been experimenting with different methods for emulating the
>> signed overflow of an 8-bit CPU. The method I've found that seems to
>> generate the most efficient code on both ARM and x86 is
>> bool overflow(unsigned int a, unsigned int b) {
>> ? ? const unsigned int sum = (int8_t)a + (int8_t)b;
>> ? ? return (int8_t)sum != sum;
>> }
> There's a GCC extension which is relevant here:
> | For conversion to a type of width N, the value is reduced modulo 2^N
> | to be within range of the type; no signal is raised.
> <>
> Using that, you can replace the final "& 0x80" with a signed
> comparison to zero, which should be give you the best possible code
> (for the generic RISC). ?You only need to hunt down a copy of Hacker's
> Delight or find the right bit twiddling by other means. 8-)

Are you thinking of something like this?

bool overflow_bit2(unsigned int a, unsigned int b) {
    const unsigned int ashift = a << 24;
    const unsigned int bshift = b << 24;
    const unsigned int sum = a + b;
    return (int)(~(a ^ b) & (a ^ sum)) < 0;

That version generates

  80:	180b      	adds	r3, r1, r0
  82:	4041      	eors	r1, r0
  84:	ea83 0200 	eor.w	r2, r3, r0
  88:	ea22 0001 	bic.w	r0, r2, r1
  8c:	0fc0      	lsrs	r0, r0, #31
  8e:	4770      	bx	lr

Whereas the unshifted version generates

  40:	180b      	adds	r3, r1, r0
  42:	4041      	eors	r1, r0
  44:	ea83 0200 	eor.w	r2, r3, r0
  48:	ea22 0001 	bic.w	r0, r2, r1
  4c:	f3c0 10c0 	ubfx	r0, r0, #7, #1
  50:	4770      	bx	lr

So maybe a bit better. (I'm no ARM pro, but the compiler does seem to
take advantage of the fact that it's testing the real sign bit at

Btw, & 0x80000000 generates the same code.


Index Nav: [Date Index] [Subject Index] [Author Index] [Thread Index]
Message Nav: [Date Prev] [Date Next] [Thread Prev] [Thread Next]