[Mesa-dev] [PATCH] nir/algebraic: Remove imprecise flog2 optimizations

[Jason Ekstrand]

While mathematically correct, these two optimizations result in an
expression with substantially lower precision than the original.  For any
positive finite floating-point value, log2(x) is well-defined and finite.
More precisely, it is in the range [-150, 150] so any sum of logarithms
log2(a) + log2(b) is also well-defined and finite as long as a and b are
both positive and finite.  However, if a and b are either very small or
very large, their product may get flushed to infinity or zero causing
log2(a * b) to be nowhere close to log2(a) + log2(b).

This imprecision was causing incorrect rendering in Talos Principal because
part of its HDR rendering process involves doing 8 texture operations,
clamping the result to [0, 65000], taking a dot-product with a constant,
and then taking the log2.  This is done 6 or 8 times and summed to produce
the final result which is written to a red texture.  In cases where you
have a region of the screen that is very dark, it can end up getting a
result value of -inf which is not what is intended.

Signed-off-by: Jason Ekstrand <jason at jlekstrand.net>
Bugzilla: https://bugs.freedesktop.org/show_bug.cgi?id=96425
Cc: Matt Turner <mattst88 at gmail.com>
---
 src/compiler/nir/nir_opt_algebraic.py | 2 --
 1 file changed, 2 deletions(-)

diff --git a/src/compiler/nir/nir_opt_algebraic.py b/src/compiler/nir/nir_opt_algebraic.py
index 011263a..fd22801 100644
--- a/src/compiler/nir/nir_opt_algebraic.py
+++ b/src/compiler/nir/nir_opt_algebraic.py
@@ -233,8 +233,6 @@ optimizations = [
    (('~flog2', ('frcp', a)), ('fneg', ('flog2', a))),
    (('~flog2', ('frsq', a)), ('fmul', -0.5, ('flog2', a))),
    (('~flog2', ('fpow', a, b)), ('fmul', b, ('flog2', a))),
-   (('~fadd', ('flog2', a), ('flog2', b)), ('flog2', ('fmul', a, b))),
-   (('~fadd', ('flog2', a), ('fneg', ('flog2', b))), ('flog2', ('fdiv', a, b))),
    (('~fmul', ('fexp2', a), ('fexp2', b)), ('fexp2', ('fadd', a, b))),
    # Division and reciprocal
    (('~fdiv', 1.0, a), ('frcp', a)),
-- 
2.5.0.400.gff86faf