Add conditional branch encodings for RISC-V.

Not all br_icmp opcodes are present in the ISA. The missing ones can be
reached by commuting operands.

Don't attempt to encode EBB offsets yet. For now just emit an EBB
relocation for the branch instruction.

lib/cretonne/meta/isa/riscv/encodings.py

@@ -5,14 +5,16 @@ from __future__ import absolute_import
 from base import instructions as base
 from base.immediates import intcc
 from .defs import RV32, RV64
-from .recipes import OPIMM, OPIMM32, OP, OP32, LUI
-from .recipes import JALR, R, Rshamt, Ricmp, I, Iicmp, Iret, U
+from .recipes import OPIMM, OPIMM32, OP, OP32, LUI, BRANCH
+from .recipes import JALR, R, Rshamt, Ricmp, I, Iicmp, Iret, U, SB
 from .settings import use_m
 from cdsl.ast import Var
 
 # Dummies for instruction predicates.
 x = Var('x')
 y = Var('y')
+dest = Var('dest')
+args = Var('args')
 
 # Basic arithmetic binary instructions are encoded in an R-type instruction.
 for inst,           inst_imm,      f3,    f7 in [
@@ -79,6 +81,18 @@ RV64.enc(base.imul.i32, R, OP32(0b000, 0b0000001), isap=use_m)
 
 # Control flow.
 
+# Conditional branches.
+for cond,           f3 in [
+        (intcc.eq,  0b000),
+        (intcc.ne,  0b001),
+        (intcc.slt, 0b100),
+        (intcc.sge, 0b101),
+        (intcc.ult, 0b110),
+        (intcc.uge, 0b111)
+        ]:
+    RV32.enc(base.br_icmp.i32(cond, x, y, dest, args), SB, BRANCH(f3))
+    RV64.enc(base.br_icmp.i64(cond, x, y, dest, args), SB, BRANCH(f3))
+
 # Returns are a special case of JALR.
 # Note: Return stack predictors will only recognize this as a return when the
 # return address is provided in `x1`. We may want a special encoding to enforce