I think we can model the signed zero problem by keeping track of a sign property, similar to how we keep track of a NAN property. The property can be yes, no, or unknown, and would apply to the entire range.
[0.0, 0.0] SIGN => -0.0 singleton [0.0, 0.0] !SIGN => +0.0 singleton [0.0, 0.0] VARYING => [-0.0, +0.0] sign unknown frange::singleton_p() would return the appropriate zero if the sign bit is definitely known, otherwise it would return NULL, which would keep VRP from propagating it. This is a sample of how I envision it working with __builtin_signbit: =========== BB 2 ============ Imports: x_3(D) Exports: _1 x_3(D) _1 : x_3(D)(I) x_3(D) [frange] float VARYING <bb 2> : _1 = __builtin_signbit (x_3(D)); if (_1 == 0) goto <bb 3>; [INV] else goto <bb 4>; [INV] 2->3 (T) _1 : [irange] int [0, 0] NONZERO 0x0 2->3 (T) x_3(D) : [frange] float [0.0, Inf] !SIGN 2->4 (F) _1 : [irange] int [-INF, -1][1, +INF] 2->4 (F) x_3(D) : [frange] float [ -Inf, 0.0] SIGN That is, on the TRUE side x_3 can be assumed to be positive, including the zero. On the FALSE side x_3 is negative, also including the zero. We can keep the endpoints in sync with the sign bit for free, since we have endpoints. So, setting the sign bit on a range to either yes or no, would automatically intersect it to [-INF, 0] or [0, +INF] respectively. With this in play, VRP could propagate a 0.0 if it knows the sign. For example: if (x == 0.0 && __builtin_signbit (x) == 0) bark(x); ...would generate: =========== BB 2 ============ Imports: x_3(D) Exports: x_3(D) x_3(D) [frange] float VARYING <bb 2> : if (x_3(D) == 0.0) goto <bb 3>; [INV] else goto <bb 5>; [INV] 2->3 (T) x_3(D) : [frange] float [0.0, 0.0] !NAN =========== BB 3 ============ Imports: x_3(D) Exports: _1 x_3(D) _1 : x_3(D)(I) x_3(D) [frange] float [0.0, 0.0] !NAN <bb 3> : _1 = __builtin_signbit (x_3(D)); if (_1 == 0) goto <bb 4>; [INV] else goto <bb 5>; [INV] 3->4 (T) _1 : [irange] int [0, 0] NONZERO 0x0 3->4 (T) x_3(D) : [frange] float [0.0, 0.0] !NAN !SIGN 3->5 (F) _1 : [irange] int [-INF, -1][1, +INF] 3->5 (F) x_3(D) : [frange] float [0.0, 0.0] !NAN SIGN =========== BB 4 ============ x_3(D) [frange] float [0.0, 0.0] !NAN !SIGN <bb 4> : bark (0.0); That is, on the 2->3 edge we know x_3 is 0.0 and !NAN, but have no information on the sign bit. Then out of BB3, we know both that x_3 is 0.0 as well as the appropriate sign. Ultimately this leads us to propagate +0.0 in BB4. I have most^Wall of it coded without regressions, with the exception of how to coerce the range-op machinery to play nice with builtins (details below). But I wanted to make sure we're all on the same page. A couple questions: Can I safely assume that +0.0 in the source (say, x = 0.0) has the sign bit cleared, and vice versa for -0.0? What's the deal with __builtin_signbit? Can I fold it to 0/1, or must I return the actual signbit, because I see differing behavior whether we fold a known value or not: abulafia:~$ cat a.c float nzero = -0.0; main(){ printf("0x%x\n", __builtin_signbit(-0.0)); printf("0x%x\n", __builtin_signbit(nzero)); } abulafia:~$ gcc a.c -w && ./a.out 0x1 0x80000000 When Andrew comes back from PTO, we'll need to talk about propagating builtins. Currently range-ops' op1_range is used to unwind back from conditionals. For example: _1 = x_9 + 5 if (_1 == 0) On the TRUE side we use op1_range to solve: 0 = x_9 + 5; We currently only handle assignments and conditionals. We would need to ability to wind back through builtins since __builtin_signbit is not part of the IL: _1 = __builtin_signbit (x_3(D)); if (_1 == 0) We have no way to augment the range for x_3 when examining the builtin. We do have a way of folding the builtin on a forward analysis, but that's a separate thing. Thoughts? Aldy On Mon, Aug 29, 2022 at 3:22 PM Jakub Jelinek <ja...@redhat.com> wrote: > > On Mon, Aug 29, 2022 at 03:13:21PM +0200, Aldy Hernandez wrote: > > It seems to me we can do this optimization regardless, but then treat > > positive and negative zero the same throughout the frange class. > > Particularly, in frange::singleton_p(). We should never return TRUE > > for any version of 0.0. This will keep VRP from propagating an > > incorrect 0.0, since all VRP does is propagate when a range is > > provably a singleton. Also, frange::zero_p() shall return true for > > any version of 0.0. > > Well, I think for HONOR_SIGNED_ZEROS it would be nice if frange was able to > differentiate between 0.0 and -0.0. > One reason is e.g. to be able to optimize copysign/signbit - if we can > prove that the sign bit on some value will be always cleared or always set, > we can fold those. > On the other side, with -fno-signed-zeros it is invalid to use > copysign/signbit on values that could be zero (well, nothing guarantees > whether the sign bit is set or clear), so for MODE_HAS_SIGNED_ZEROS && > !HONOR_SIGNED_ZEROS it is best to treat contains_p as {-0.0,0.0} being > one thing (just not singleton_p) and not bother with details like whether > a range ends or starts with -0.0 or 0.0, either of them would work the same. > And for !MODE_HAS_SIGNED_ZEROS, obviously 0.0 can be singleton_p. > > Jakub >