Add RISC-V encodings for brz and brnz.

These branches compare a register to zero. RISC-V implements this with
the %x0 hard-coded zero register.

filetests/isa/riscv/binary32.cton

@@ -91,5 +91,10 @@ ebb0:
     ; bgeu
     br_icmp uge, v1, v2, ebb0           ; bin: Branch(ebb0) 01557063
 
+    ; beq x, %x0
+    brz v1, ebb0                        ; bin: Branch(ebb0) 00050063
+    ; bne x, %x0
+    brnz v1, ebb0                       ; bin: Branch(ebb0) 00051063
+
     return
 }

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

@@ -6,7 +6,7 @@ from base import instructions as base
 from base.immediates import intcc
 from .defs import RV32, RV64
 from .recipes import OPIMM, OPIMM32, OP, OP32, LUI, BRANCH
-from .recipes import JALR, R, Rshamt, Ricmp, I, Iicmp, Iret, U, SB
+from .recipes import JALR, R, Rshamt, Ricmp, I, Iicmp, Iret, U, SB, SBzero
 from .settings import use_m
 from cdsl.ast import Var
 
@@ -93,6 +93,15 @@ for cond,           f3 in [
     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))
 
+for inst,           f3 in [
+        (base.brz,  0b000),
+        (base.brnz, 0b001)
+        ]:
+    RV32.enc(inst.i32, SBzero, BRANCH(f3))
+    RV64.enc(inst.i64, SBzero, BRANCH(f3))
+    RV32.enc(inst.b1, SBzero, BRANCH(f3))
+    RV64.enc(inst.b1, SBzero, 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

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

@@ -12,7 +12,7 @@ from __future__ import absolute_import
 from cdsl.isa import EncRecipe
 from cdsl.predicates import IsSignedInt
 from base.formats import Binary, BinaryImm, MultiAry, IntCompare, IntCompareImm
-from base.formats import UnaryImm, BranchIcmp
+from base.formats import UnaryImm, BranchIcmp, Branch
 from .registers import GPR
 
 # The low 7 bits of a RISC-V instruction is the base opcode. All 32-bit
@@ -116,3 +116,6 @@ U = EncRecipe(
 # TODO: These instructions have a +/- 4 KB branch range. How to encode that
 # constraint?
 SB = EncRecipe('SB', BranchIcmp, ins=(GPR, GPR), outs=())
+
+# SB-type branch instruction with rs2 fixed to zero.
+SBzero = EncRecipe('SBzero', Branch, ins=(GPR), outs=())

lib/cretonne/src/isa/riscv/binemit.rs

@@ -246,3 +246,16 @@ fn recipe_sb<CS: CodeSink + ?Sized>(func: &Function, inst: Inst, sink: &mut CS)
         panic!("Expected BranchIcmp format: {:?}", func.dfg[inst]);
     }
 }
+
+fn recipe_sbzero<CS: CodeSink + ?Sized>(func: &Function, inst: Inst, sink: &mut CS) {
+    if let InstructionData::Branch { destination, ref args, .. } = func.dfg[inst] {
+        let args = &args.as_slice(&func.dfg.value_lists)[0..1];
+        sink.reloc_ebb(RelocKind::Branch.into(), destination);
+        put_sb(func.encodings[inst].bits(),
+               func.locations[args[0]].unwrap_reg(),
+               0,
+               sink);
+    } else {
+        panic!("Expected Branch format: {:?}", func.dfg[inst]);
+    }
+}