[Mesa-dev] [RFC PATCH] Add GL_MESA_ieee_fp_alu_mode specification draft
imirkin at alum.mit.edu
Mon Feb 24 01:57:34 UTC 2020
We talked about something like this a while back, but the end result
was inconclusive. I added a TGSI MUL_ZERO_WINS shader property for nine.
But it'd be nice for wine to be able to control this too.
I couldn't actually find any evidence of the discussion from 2017 or so,
so ... let's have another one.
docs/specs/MESA_ieee_fp_alu_mode.spec | 136 ++++++++++++++++++++++++++
1 file changed, 136 insertions(+)
create mode 100644 docs/specs/MESA_ieee_fp_alu_mode.spec
diff --git a/docs/specs/MESA_ieee_fp_alu_mode.spec b/docs/specs/MESA_ieee_fp_alu_mode.spec
new file mode 100644
@@ -0,0 +1,136 @@
+ Ilia Mirkin, ilia 'at' x.org
+ No known IP issues.
+ OpenGL 3.0 or OpenGL ES 3.0 is required.
+ The extension is written against the OpenGL GL 3.0 and OpenGL ES 3.0
+ Pre-GL3 hardware did not generally have full IEEE floating point operation
+ support. Among other things, 0 * Infinity would work out to 0, and NaN's
+ might not be generated, or otherwise be treated improperly. GL3-class and
+ later hardware introduced full IEEE FP support, including NaN, Infinity,
+ and the proper generation of these.
+ Some software targeted at older hardware makes assumptions about how the
+ shader ALU works. And to accomodate these, GL3-class hardware has a way to
+ change how the shader ALU behaves. There are no standards around this, and
+ different hardware has different ways of dealing with it. However these
+ modes were designed specifically with such older software in mind.
+ This extension introduces a way to configure a context to be in non-IEEE
+ ALU mode. This extension does not specify precisely what this means, as
+ each vendor has something different. Generally it means non-IEEE compliant
+ handling of multiplication, as well as any other unspecified changes.
+ Accepted by the <cap> parameter of Enable, Disable, and IsEnabled, by
+ the <pname> parameter of GetBooleanv, GetIntegerv, GetFloatv, and
+ IEEE_FP_ALU_MODE_MESA 0x????
+Changes to GLSL Section 4.1.4 Floats:
+ Add the following paragraph:
+ In case that the shader is being executed in a context with
+ IEEE_FP_ALU_MODE_MESA disabled, multiplication shall produce the following
+ (non-IEEE-complaint) result:
+ float a = 0;
+ float b = Infinity;
+ float c = a * b; // c == 0
+ There may be other implications from this mode being enabled, including
+ clamping of non-finite values, or anything else the hardware mode happens
+ to enable to achieve compatibility.
+ (add to table 6.52, Miscellaneous, p.392)
+ Get Value Type Get Command Value Description Sec. Attribute
+ --------------------- ----- ----------- ------- ------------------ ------ ---------
+ IEEE_FP_ALU_MODE_MESA B IsEnabled TRUE Whether shader ALU enable
+ is in IEEE FP mode
+ (1) This specification does not precisely specify what non-IEEE FP mode is.
+ RESOLVED. Shipping hardware has different ways of dealing with it. For
+ example, Intel clamps all values. NVIDIA Tesla series has a
+ context-wide mode for controlling whether zero wins in multiplication
+ or follows IEEE rules. NVIDIA Fermi+ series as well as ATI/AMD Radeon
+ R600+ has separate opcodes which control this (but again, a different
+ set of operations are covered).
+ A single extension which is going to be easy to use for emulation
+ software is thus much harder to write if it's to precisely specify
+ The applications that want these have already been written and tested
+ against these approaches, so we know they all work with whatever the
+ hardware has to offer.
+ (2) Why use an Enable instead of a shader layout token?
+ RESOLVED. Because some hardware implementations don't allow
+ controlling this on a per-stage level. While one could come up with
+ rules requiring linked program stages to have the same setting, this
+ is going to be extra validation for the implementations to
+ implement. Furthermore, one would want these rules to also apply to
+ fixed-function-generated shaders equally. Instead a simple mode should
+ be able to flip this on and off.
+ (3) What about FP denorms?
+ RESOLVED. The same hardware tends to also have a way to control
+ whether denorm FP values are flushed to zero. GLSL does not specify
+ this explicitly, but some software relies on denorms being
+ flushed. Should there be a desire to allow denorms to work, this can
+ be done by another extension.
+ (4) What is the expected usage for this?
+ RESOLVED. Software which enables older games to operate,
+ e.g. emulators, will now be able to do shader translation without
+ copious checks for these "error" conditions.
+ Revision 1, ilia, 2020-02-23
+ - Initial draft
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