From: Luke Kenneth Casson Leighton <lkcl@lkcl.net>
Date: Sun, 28 Jul 2019 12:32:59 +0000 (+0100)
Subject: add code comments
X-Git-Tag: ls180-24jan2020~700
X-Git-Url: https://git.libre-soc.org/?a=commitdiff_plain;h=02dbf3a7d73a160f31cd04142f086c125990d2ed;p=ieee754fpu.git

add code comments
---

diff --git a/src/ieee754/fclass/fclass.py b/src/ieee754/fclass/fclass.py
index e1b034e6..f50412c4 100644
--- a/src/ieee754/fclass/fclass.py
+++ b/src/ieee754/fclass/fclass.py
@@ -54,6 +54,9 @@ class FPClassMod(Elaboratable):
         m.d.comb += is_sig_nan.eq(a1.exp_128 & (msbzero) & (~a1.m_zero))
         subnormal = a1.exp_n127
 
+        # this is hardware-optimal but very hard to understand.
+        # see unit test test_fclass_pipe.py fclass() for what's
+        # going on.
         m.d.comb += self.o.z.eq(Cat(
                     a1.s   & a1.is_inf,                 # | −inf.
                     a1.s   & finite_nzero & ~subnormal, # | -normal number.
diff --git a/src/ieee754/fclass/test/test_fclass_pipe.py b/src/ieee754/fclass/test/test_fclass_pipe.py
index b96d17ff..30fcf4e8 100644
--- a/src/ieee754/fclass/test/test_fclass_pipe.py
+++ b/src/ieee754/fclass/test/test_fclass_pipe.py
@@ -10,6 +10,10 @@ from sfpy import Float64, Float32, Float16
 
 def fclass(wid, x):
     """ analyses the FP number and returns a RISC-V "FCLASS" unary bitfield
+
+        this is easy to understand however it has redundant checks (which
+        don't matter because performance of *testing* is not hardware-critical)
+        see FPClassMod for a hardware-optimal (hard-to-read) version
     """
     x = x.bits
     fmt = FPFormat.standard(wid)