Encode iconst.i32 for RISC-V.

For large constants with the low 12 bits clear, we already have the
"lui" encoding. Add "addi %x0" encodings for signed 12-bit constants.

filetests/isa/riscv/binary32.cton

@@ -77,6 +77,9 @@ ebb0(v9999: i32):
     ; lui
     [-,%x7]     v140 = iconst.i32 0x12345000            ; bin: 123453b7
     [-,%x16]    v141 = iconst.i32 0xffffffff_fedcb000   ; bin: fedcb837
+    ; addi
+    [-,%x7]     v142 = iconst.i32 1000                  ; bin: 3e800393
+    [-,%x16]    v143 = iconst.i32 -905                  ; bin: c7700813
 
     ; Copies alias to iadd_imm.
     [-,%x7]     v150 = copy v1                          ; bin: 00050393

filetests/isa/riscv/verify-encoding.cton

@@ -6,7 +6,7 @@ function %RV32I(i32 link [%x1]) -> i32 link [%x1] {
 
 ebb0(v9999: i32):
     ; iconst.i32 needs legalizing, so it should throw a
-    [R#0,-]         v1 = iconst.i32 1 ; error: Instruction failed to re-encode
+    [R#0,-]         v1 = iconst.i32 0xf0f0f0f0f0 ; error: Instruction failed to re-encode
     return v9999
 }
 

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

@@ -7,7 +7,7 @@ from base.immediates import intcc
 from .defs import RV32, RV64
 from .recipes import OPIMM, OPIMM32, OP, OP32, LUI, BRANCH, JALR, JAL
 from .recipes import LOAD, STORE
-from .recipes import R, Rshamt, Ricmp, I, Iicmp, Iret, Icall, Icopy
+from .recipes import R, Rshamt, Ricmp, I, Iz, Iicmp, Iret, Icall, Icopy
 from .recipes import U, UJ, UJcall, SB, SBzero, GPsp, GPfi
 from .settings import use_m
 from cdsl.ast import Var
@@ -40,6 +40,11 @@ RV64.enc(base.isub.i32, R, OP32(0b000, 0b0100000))
 # There are no andiw/oriw/xoriw variations.
 RV64.enc(base.iadd_imm.i32, I, OPIMM32(0b000))
 
+# Use iadd_imm with %x0 to materialize constants.
+RV32.enc(base.iconst.i32, Iz, OPIMM(0b000))
+RV64.enc(base.iconst.i32, Iz, OPIMM(0b000))
+RV64.enc(base.iconst.i64, Iz, OPIMM(0b000))
+
 # Dynamic shifts have the same masking semantics as the cton base instructions.
 for inst,           inst_imm,      f3,    f7 in [
         (base.ishl, base.ishl_imm, 0b001, 0b0000000),

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

@@ -104,6 +104,11 @@ I = EncRecipe(
         'I', BinaryImm, size=4, ins=GPR, outs=GPR,
         instp=IsSignedInt(BinaryImm.imm, 12))
 
+# I-type instruction with a hardcoded %x0 rs1.
+Iz = EncRecipe(
+        'Iz', UnaryImm, size=4, ins=(), outs=GPR,
+        instp=IsSignedInt(UnaryImm.imm, 12))
+
 # I-type encoding of an integer comparison.
 Iicmp = EncRecipe(
         'Iicmp', IntCompareImm, size=4, ins=GPR, outs=GPR,

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

@@ -160,6 +160,18 @@ fn recipe_i<CS: CodeSink + ?Sized>(func: &Function, inst: Inst, sink: &mut CS) {
     }
 }
 
+fn recipe_iz<CS: CodeSink + ?Sized>(func: &Function, inst: Inst, sink: &mut CS) {
+    if let InstructionData::UnaryImm { imm, .. } = func.dfg[inst] {
+        put_i(func.encodings[inst].bits(),
+              0,
+              imm.into(),
+              func.locations[func.dfg.first_result(inst)].unwrap_reg(),
+              sink);
+    } else {
+        panic!("Expected UnaryImm format: {:?}", func.dfg[inst]);
+    }
+}
+
 fn recipe_iicmp<CS: CodeSink + ?Sized>(func: &Function, inst: Inst, sink: &mut CS) {
     if let InstructionData::IntCompareImm { arg, imm, .. } = func.dfg[inst] {
         put_i(func.encodings[inst].bits(),