[vm, compiler] Implement Smi_bitLength intrinsic on RV.

TEST=ci
Change-Id: Ica3cace6cf6ac416e9db8c1b682df429aaf595e4
Reviewed-on: https://dart-review.googlesource.com/c/sdk/+/244740
Commit-Queue: Ryan Macnak <rmacnak@google.com>
Reviewed-by: Alexander Markov <alexmarkov@google.com>

Author: rmacnak-google

runtime/vm/compiler/asm_intrinsifier_riscv.cc

@@ -151,8 +151,19 @@ void AsmIntrinsifier::Integer_equal(Assembler* assembler,
 
 void AsmIntrinsifier::Smi_bitLength(Assembler* assembler,
                                     Label* normal_ir_body) {
-  // TODO(riscv)
-  __ Bind(normal_ir_body);
+  __ lx(A0, Address(SP, 0 * target::kWordSize));
+  __ SmiUntag(A0);
+
+  // XOR with sign bit to complement bits if value is negative.
+  __ srai(A1, A0, XLEN - 1);
+  __ xor_(A0, A0, A1);
+
+  __ CountLeadingZeroes(A0, A0);
+
+  __ li(TMP, XLEN);
+  __ sub(A0, TMP, A0);
+  __ SmiTag(A0);
+  __ ret();
 }
 
 void AsmIntrinsifier::Bigint_lsh(Assembler* assembler, Label* normal_ir_body) {

runtime/vm/compiler/assembler/assembler_riscv.cc

@@ -4470,6 +4470,56 @@ void Assembler::MultiplyBranchOverflow(Register rd,
   bne(TMP, TMP2, overflow);
 }
 
+void Assembler::CountLeadingZeroes(Register rd, Register rs) {
+  // Note: clz will appear in the Zbb extension.
+  // if (Supports(RV_Zbb)) {
+  //   clz(rd, rs);
+  // }
+
+  //  n = XLEN
+  //  y = x >>32; if (y != 0) { n = n - 32; x = y; }
+  //  y = x >>16; if (y != 0) { n = n - 16; x = y; }
+  //  y = x >> 8; if (y != 0) { n = n - 8; x = y; }
+  //  y = x >> 4; if (y != 0) { n = n - 4; x = y; }
+  //  y = x >> 2; if (y != 0) { n = n - 2; x = y; }
+  //  y = x >> 1; if (y != 0) { return n - 2; }
+  //  return n - x;
+  Label l0, l1, l2, l3, l4, l5;
+  li(TMP2, XLEN);
+#if XLEN == 64
+  srli(TMP, rs, 32);
+  beqz(TMP, &l0, Assembler::kNearJump);
+  subi(TMP2, TMP2, 32);
+  mv(rs, TMP);
+  Bind(&l0);
+#endif
+  srli(TMP, rs, 16);
+  beqz(TMP, &l1, Assembler::kNearJump);
+  subi(TMP2, TMP2, 16);
+  mv(rs, TMP);
+  Bind(&l1);
+  srli(TMP, rs, 8);
+  beqz(TMP, &l2, Assembler::kNearJump);
+  subi(TMP2, TMP2, 8);
+  mv(rs, TMP);
+  Bind(&l2);
+  srli(TMP, rs, 4);
+  beqz(TMP, &l3, Assembler::kNearJump);
+  subi(TMP2, TMP2, 4);
+  mv(rs, TMP);
+  Bind(&l3);
+  srli(TMP, rs, 2);
+  beqz(TMP, &l4, Assembler::kNearJump);
+  subi(TMP2, TMP2, 2);
+  mv(rs, TMP);
+  Bind(&l4);
+  srli(TMP, rs, 1);
+  sub(rd, TMP2, rs);
+  beqz(TMP, &l5, Assembler::kNearJump);
+  subi(rd, TMP2, 2);
+  Bind(&l5);
+}
+
 }  // namespace compiler
 
 }  // namespace dart

runtime/vm/compiler/assembler/assembler_riscv.h

@@ -1412,6 +1412,9 @@ class Assembler : public MicroAssembler {
                               Register rs2,
                               Label* overflow);
 
+  // Clobbers [rs].
+  void CountLeadingZeroes(Register rd, Register rs);
+
  private:
   bool constant_pool_allowed_;