Adding instructions to the Simplscalar decode tree
The simplscalar decode tree for instructions can be at first quiet daunting.
Tracing the decode pattern is a pen and paper exercise.
e.g the range of instructions that start with 0×06 are defined as
DEFLINK(MEMREGPOST_LINK, 0×06, “memregpost_link”, 20, 0x0f)
CONNECT(MEMREGPOST_LINK)
This means shift the inst that start with 0×06 e.g 0×06000000 and examine bits 20 – 23
After the connect code we have an entry for 0×00 this means that if bits 20 -23 are 00 this will match and generate an opcode for this instruction
e.g 0×06000000
#define STR_R_IMPL \
etc etc
DEFINST(STR_R, 0×00,
etc
The challenge facing anyone is to add extra opcodes that are meaningful.
If one examines the 0×06 range of opcodes it is evident that instructions such as
0×06000100, 0×06000200 are unused.
So how do we use them in Simplescalar?
Well what we need to do is modify arm.def to examine bits 8 – 11 first, if these bits are 0 then they are the original instructions defined under memregpost. If not they are our new instructions.
A statement such as the one below takes care of examining bits 8 -11 of an inst that starts with 0×06
DEFLINK(MEMREGPOST_LINK, 0×06, “memregpost_link”, 8, 0x0f)
CONNECT(MEMREGPOST_LINK)
We now need to ensure that 0×06 instructions whose bits are 0 from pos 8 to 11 are directed to a different part of the decode tree. The deflink code below will be executed if the bits are 0 and will examine bits 20 – 23 of the 0×06 inst in the connect section.
In the example below inst 0×06000000 will be associated with a deflink to a new tree where the isnt str%c will be associated with 0×06000000. While inst 0×06000100 will be associated with the inst demo because bits 8 to 11 equate to 1
DEFLINK(MEMREGPOSTNEW_LINK, 0×00, “memregpostnew_link”, 20, 0x0f)
#define demo_IMPL \
etc
DEFINST(demo, 0×01,
“demo%c”, “%d,%n,%m”,
etc
CONNECT(MEMREGPOSTNEW_LINK)
#define STR_R_IMPL \
etc
DEFINST(STR_R, 0×00,
“str%c”, “%d,[%n],-%m!”,
etc
We also need to to add these new opcodes to your gas see http://www-unix.ecs.umass.edu/~yhan/ for more details on how to hack GAS.
http://www.irishsilicon.com/tag/simplescalar-decode-tree-add-instruction-arm-def/
