TCG(Tiny Code Generator)

QEMU support multiple accel (accelerator, determine how the vCPU to exec)

• simulator (TCG)
• virtual (KVM, HVF) We pay attention to the TCG technology.

TCG IR

Guest  Inst
    |     
    v     
 QEMU IR  
    |     
    v     
Host inst 

• decode: pick up the rd rs1 rs2 filed, pattern specific instrucment
• translate: call trans_add() function, generate TCG IR
• exec: TCG backend translate IR to host instrument and exec

Decodetree is glue between step one and step two:

guest instn (32-bit binary)
     |
     v
decode_insn32()     // decodetree.py generated
   1. match bit (bitmask): determine which instn
   2. pick up field: fill the arg struct
   3. call trans_xxx(ctx, arg_xxx *a)
     |
     v
trans_add(ctx, arg_r *a) // developer writed function
	read rs1, rs2 -> generate TCG add op -> write back rs
	 |
	 v
TCG backend compile to host instn and exec 

+----------------+           +---------------+            +-----------------------+
| target/arch/   |  input    | scripts/      |  output    | decode-@BASENAME@     |
| insn32.decode  +---------->| decodetree.py +----------->|     .c.inc            |
+----------------+           +---------------+            +-----------------------+

reference

• https://qemu.gevico.online/tutorial/2026/ch2/qemu-tcg/#tcg_1
• https://www.qemu.org/docs/master/devel/decodetree.html