Support software emulation for multiplication on RISC-V targets

Previously, shecc would just generate hardware multiplication
instructions for multiplication operations.

This commit adjusts riscv-codegen.c so that shecc can generate
a sequence of instructions to achieve multiplication without
any RV32M instruction if the '+m' option is disable.

src/riscv-codegen.c

@@ -50,7 +50,6 @@ void update_elf_offset(ph2_ir_t *ph2_ir)
     case OP_indirect:
     case OP_add:
     case OP_sub:
-    case OP_mul:
     case OP_div:
     case OP_mod:
     case OP_lshift:
@@ -65,6 +64,12 @@ void update_elf_offset(ph2_ir_t *ph2_ir)
     case OP_bit_not:
         elf_offset += 4;
         return;
+    case OP_mul:
+        if (hard_mul_div)
+            elf_offset += 4;
+        else
+            elf_offset += 52;
+        return;
     case OP_load_data_address:
     case OP_neq:
     case OP_geq:
@@ -283,7 +288,23 @@ void emit_ph2_ir(ph2_ir_t *ph2_ir)
         emit(__sub(rd, rs1, rs2));
         return;
     case OP_mul:
-        emit(__mul(rd, rs1, rs2));
+        if (hard_mul_div)
+            emit(__mul(rd, rs1, rs2));
+        else {
+            emit(__addi(__t0, __zero, 0));
+            emit(__addi(__t1, __zero, 0));
+            emit(__addi(__t3, rs1, 0));
+            emit(__addi(__t4, rs2, 0));
+            emit(__beq(__t3, __zero, 32));
+            emit(__beq(__t4, __zero, 28));
+            emit(__andi(__t1, __t4, 1));
+            emit(__beq(__t1, __zero, 8));
+            emit(__add(__t0, __t0, __t3));
+            emit(__slli(__t3, __t3, 1));
+            emit(__srli(__t4, __t4, 1));
+            emit(__jal(__zero, -28));
+            emit(__addi(rd, __t0, 0));
+        }
         return;
     case OP_div:
         emit(__div(rd, rs1, rs2));

tests/driver.sh

@@ -592,6 +592,39 @@ int main()
 }
 EOF
 
+# Multiplication for signed integers
+try_output 0 "35 -35 -35 35" << EOF
+int main()
+{
+    printf("%d %d %d %d\n", 5 * 7, 5 * -7, -5 * 7, -5 * -7);
+    return 0;
+}
+EOF
+
+try_output 0 "-212121 -535050 336105 666666666" << EOF
+int main()
+{
+    printf("%d %d %d %d\n", -333 * 637, 1450 * -369, 37345 * 9, -111111111 * -6);
+    return 0;
+}
+EOF
+
+try_output 0 "1073676289 -131071 30" << EOF
+int main()
+{
+    printf("%d %d %d\n", 32767 * 32767, 65535 * 65535, 54 * 5 * 954437177);
+    return 0;
+}
+EOF
+
+try_output 0 "-2 6 24" << EOF
+int main()
+{
+    printf("%d %d %d\n", -1 * 2, -1 * 2 * -3, -1 * 2 * -3 * 4);
+    return 0;
+}
+EOF
+
 # Division and modulo for signed integers
 try_output 0 "-1 -2" << EOF
 int main()