On Fri, Jul 18, 2014 at 9:19 AM, Ilia Mirkin <imir...@alum.mit.edu> wrote: > The current code appears to work in simple tests, however this will > guarantee that the returned exponent is 0 for a 0 value. > > Signed-off-by: Ilia Mirkin <imir...@alum.mit.edu> > --- > > I couldn't make a simple test-case that would cause the current logic to > fail. However when I did the same thing with doubles, I ran into trouble. It > seems safer to move the csel outside of the add in case the value actually has > a non-0 exponent despite a 0 significand. > > src/glsl/builtin_functions.cpp | 4 ++-- > 1 file changed, 2 insertions(+), 2 deletions(-) > > diff --git a/src/glsl/builtin_functions.cpp b/src/glsl/builtin_functions.cpp > index e01742c..5755de9 100644 > --- a/src/glsl/builtin_functions.cpp > +++ b/src/glsl/builtin_functions.cpp > @@ -4229,8 +4229,8 @@ builtin_builder::_frexp(const glsl_type *x_type, const > glsl_type *exp_type) > * to unsigned integers to ensure that 1 bits aren't shifted in. > */ > body.emit(assign(exponent, rshift(bitcast_f2i(abs(x)), exponent_shift))); > - body.emit(assign(exponent, add(exponent, csel(is_not_zero, exponent_bias, > - imm(0, vec_elem))))); > + body.emit(assign(exponent, csel(is_not_zero, add(exponent, exponent_bias), > + imm(0, vec_elem))));
So you're changing the logic from exponent = (f2i(abs(x) >> 23) + (x != 0.0f) ? -126 : 0; to exponent = (x != 0.0f) ? (f2i(abs(x) >> 23) - 126 : 0; These seem identical to me, and trivially so for 0.0f/-0.0f. I have a feeling that this patch is papering over a bug in your code generation for f2i(abs(x)). In commit 9c48ae75 I fixed a bug in i965 where instead of generating f2i(abs(x)) we'd apply the source modifier effectively after the bitcast (reading the register with a different type), so we'd get abs(f2i(x)). For 0.0f, applying the f2i and abs out of order doesn't affect the result, but for -0.0f (0x80000000, -2147483648) instead of getting 0, you'd get abs(-2147483648) (which is likely -2147483648 itself!). I don't have any idea about why this might occur for you with doubles but not floats, but you should have something to look into now. _______________________________________________ mesa-dev mailing list mesa-dev@lists.freedesktop.org http://lists.freedesktop.org/mailman/listinfo/mesa-dev
