[libre-riscv-dev] div/mod algorithm written in python

[Luke Kenneth Casson Leighton]

i have absooolutely noooo idea why these "magic constants" fix things
when the log2_radix is changed!

i've set up a test run with log2_radix=3 (radix=8) and with the number
of combinatorial stages set to 3 "major" stages. for FP32 that turns
into a 6 stage pipeline which is just about tolerable (4 DIV stages,
one de-normalisation etc., one post-normalisation etc.).  numbering
there goes from 0-9 on the calculate stages, so that's 10 total:
3x10=30 bits.  probably room to reduce that.

for FP64 it's 9 stages: 7 DIVs, one de-norm, one post-norm.  the
numbering on the n_stages goes from 0-18 so 19 stages @ 3 bits each
(log2_radix=3) that's 57 bits.

for FP16 numbering is going 0-4 so that's 5 calculate stages, 5x3 = 15 bits.

l.

diff --git a/src/ieee754/fpdiv/div0.py b/src/ieee754/fpdiv/div0.py
index 80ffc68..8735533 100644
--- a/src/ieee754/fpdiv/div0.py
+++ b/src/ieee754/fpdiv/div0.py
@@ -55,11 +55,24 @@ class FPDivStage0Mod(Elaboratable):
             # into DivPipeInputData dividend and divisor.

             if self.pspec.width == 16:
-                extra = 3
+                if self.pspec.log2_radix == 1:
+                    extra = 2
+                elif self.pspec.log2_radix == 3:
+                    extra = 2
+                else:
+                    extra = 3
             elif self.pspec.width == 32:
-                extra = 4
+                if self.pspec.log2_radix == 1:
+                    extra = 3
+                else:
+                    extra = 4
             elif self.pspec.width == 64:
-                extra = 3
+                if self.pspec.log2_radix == 1:
+                    extra = 2
+                elif self.pspec.log2_radix == 3:
+                    extra = 2
+                else:
+                    extra = 3
             # the mantissas, having been de-normalised (and containing
             # a "1" in the MSB) represent numbers in the range 0.5 to
             # 0.9999999-recurring.  the min and max range of the