Optimize assembly dynamic jump

core/src/riscv2zisk_context.rs

@@ -75,6 +75,9 @@ const FLOAT_HANDLER_RETURN_ADDR: u64 = FLOAT_HANDLER_ADDR + 4 * 34; // 31 regs +
 /// Mask to apply to the target address of JALR instructions, to ensure the least significant bit is 0
 const JALR_MASK: u64 = 0xfffffffffffffffe;
 
+#[cfg(not(feature = "float"))]
+const NO_FLOAT_ECALL_ADDR: u64 = ROM_EXIT + 0x58;
+
 /// Context to store the list of converted ZisK instructions, including their program address and a
 /// map to store the instructions
 pub struct Riscv2ZiskContext<'a> {
@@ -1261,7 +1264,15 @@ impl Riscv2ZiskContext<'_> {
     pub fn ecall(&mut self, i: &RiscvInstruction) {
         let mut zib = ZiskInstBuilder::new_from_riscv(i.rom_address, i.inst.clone());
         zib.src_a("imm", 0, false);
+        // If the float feature is enabled, we use the MTVEC register as the address to jump to for
+        // the ecall. If not, we use a fixed address (NO_FLOAT_ECALL_ADDR) for the ecall. This
+        // allows us to handle system calls differently based on whether floating-point operations
+        // are supported or not.  When float is disabled, this avoids the only dynamic jump to the
+        // lower addresses space, improving the performance of dynamic jumps in general.
+        #[cfg(feature = "float")]
         zib.src_b("mem", MTVEC, false);
+        #[cfg(not(feature = "float"))]
+        zib.src_b("imm", NO_FLOAT_ECALL_ADDR, false);
         zib.op("copyb").unwrap();
         zib.store_pc("reg", 1, false);
         zib.set_pc();
@@ -2432,7 +2443,11 @@ pub fn add_entry_exit_jmp(rom: &mut ZiskRom, addr: u64) {
 
     // Calculate the trap handler rom pc address as an offset from the current instruction address
     // to the beginning of the ecall section
+    #[cfg(not(feature = "float"))]
+    assert!(rom.next_init_inst_addr == ROM_EXIT + 4);
     let trap_handler: u64 = rom.next_init_inst_addr + 0x54;
+    #[cfg(not(feature = "float"))]
+    assert!(trap_handler == NO_FLOAT_ECALL_ADDR);
 
     // :0000 we note the rom pc address offset from the first address for each instruction
     // Store the Zisk architecture ID into memory

core/src/zisk_rom_2_asm.rs

@@ -7907,8 +7907,11 @@ impl ZiskRom2Asm {
             high_address,
             ctx.comment_str("is pc a low address?")
         );
-        *code += &format!("\tcmp {REG_PC}, {REG_ADDRESS}\n");
-        *code += &format!("\tjb pc_{:x}_jump_to_low_address\n", ctx.pc);
+        #[cfg(feature = "float")]
+        {
+            *code += &format!("\tcmp {REG_PC}, {REG_ADDRESS}\n");
+            *code += &format!("\tjb pc_{:x}_jump_to_low_address\n", ctx.pc);
+        }
         *code +=
             &format!("\tsub {}, {} {}\n", REG_PC, REG_ADDRESS, ctx.comment_str("pc -= ROM_ADDR"));
         *code += &format!(
@@ -7925,21 +7928,24 @@ impl ZiskRom2Asm {
             ctx.comment_str("address = map[pc]")
         );
         *code += &format!("\tjmp {} {}\n", REG_ADDRESS, ctx.comment_str("jump to address"));
-        *code += &format!("pc_{:x}_jump_to_low_address:\n", ctx.pc);
-        *code += &format!("\tsub {}, 0x1000 {}\n", REG_PC, ctx.comment_str("pc -= ROM_ENTRY"));
-        *code += &format!(
-            "\tlea {}, [map_pc_1000] {}\n",
-            REG_ADDRESS,
-            ctx.comment_str("address = map[ROM_ENTRY]")
-        );
-        *code += &format!(
-            "\tmov {}, [{} + {}*2] {}\n",
-            REG_ADDRESS,
-            REG_ADDRESS,
-            REG_PC,
-            ctx.comment_str("address = map[pc]")
-        );
-        *code += &format!("\tjmp {} {}\n", REG_ADDRESS, ctx.comment_str("jump to address"));
+        #[cfg(feature = "float")]
+        {
+            *code += &format!("pc_{:x}_jump_to_low_address:\n", ctx.pc);
+            *code += &format!("\tsub {}, 0x1000 {}\n", REG_PC, ctx.comment_str("pc -= ROM_ENTRY"));
+            *code += &format!(
+                "\tlea {}, [map_pc_1000] {}\n",
+                REG_ADDRESS,
+                ctx.comment_str("address = map[ROM_ENTRY]")
+            );
+            *code += &format!(
+                "\tmov {}, [{} + {}*2] {}\n",
+                REG_ADDRESS,
+                REG_ADDRESS,
+                REG_PC,
+                ctx.comment_str("address = map[pc]")
+            );
+            *code += &format!("\tjmp {} {}\n", REG_ADDRESS, ctx.comment_str("jump to address"));
+        }
     }
 
     /*************/