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

[Luke Kenneth Casson Leighton]

jacob what's the setup for RSQRT, i'm getting consistent answers of zero.

changing this from 3 to 2 gives some non-zero answers

        with m.Else():  # DivPipeCoreOperation.RSqrtRem
            m.d.comb += self.o.compare_lhs.eq(
                1 << (self.core_config.fract_width * 3))

--- a/src/ieee754/fpdiv/div0.py
+++ b/src/ieee754/fpdiv/div0.py
@@ -114,6 +114,21 @@ class FPDivStage0Mod(Elaboratable):
                              self.o.operation.eq(Const(1)) # XXX SQRT operation
                     ]

+            # RSQRT
+            with m.Elif(self.i.ctx.op == 2):
+                am0 = Signal(len(self.i.a.m)+3, reset_less=True)
+                with m.If(self.i.a.e[0]):
+                    m.d.comb += am0.eq(Cat(self.i.a.m, 0)<<(extra-2))
+                    m.d.comb += self.o.z.e.eq(-((self.i.a.e+1) >> 1)+1)
+                with m.Else():
+                    m.d.comb += am0.eq(Cat(0, self.i.a.m)<<(extra-2))
+                    m.d.comb += self.o.z.e.eq((self.i.a.e >> 1)+1)
+
+                m.d.comb += [self.o.z.s.eq(self.i.a.s),
+                             self.o.divisor_radicand.eq(am0),
+                             self.o.operation.eq(Const(2)) # XXX
RSQRT operation
+                    ]
+

On Tue, Jul 23, 2019 at 10:30 PM Luke Kenneth Casson Leighton
<lkcl at lkcl.net> wrote:
>
> ok so i managed to indentify that pass_flags is unary-encoded and is a
> priority unary-encoding.  that means that PriorityEncoder can be used
> to turn pass_flags (unary) into next_bits (binary), and that a nmigen
> Array can then be used to select the compare_rhs.  yosys knows to use
> a "PMUX" for this.
>
> looks much cleaner code, and the graphviz is a *lot* cleaner.  all the
> XORs are gone.   by also moving stuff to a new class, Trial, a whooole
> stack of magic constants that were a dog's dinner mess are also gone
> from the top-level, and each "Trial" instance is now way, waaay
> cleaner as well.
>
> l.