I am looking for guidance on how to:
1. At compile time: ensure that a jump table is generated for a *specific* switch instruction
2. At runtime: get hold of the jump table entries for a specific pair of values and swap them.
Let's say I have:
switch i32 %2, label %7 [
i32 0, label %3
i32 1, label %4
i32 2, label %5
i32 3, label %6
]
:
:
for which the following assembly code gets generated:
jmpq *.LJTI0_0(,%rdi,8)
:
.LJTI0_0:
.quad .LBB0_2
.quad .LBB0_3
.quad .LBB0_4
.quad .LBB0_5
Based on some run-time conditions, I may want to change the behavior of the switch instruction by swapping the jump table entries for 0 and 2 at runtime. My jump table should then become:
.LJTI0_0:
.quad .LBB0_4
.quad .LBB0_3
.quad .LBB0_2
.quad .LBB0_5
As this is a very simple transformation, I don't want to incur the overhead of recompiling the entire function at run-time.
My target architectures are x86 and x86_64. I understand that the above transformation may not be safe in general, but I know that for the specific switch instructions that I am targeting, it is safe. I am not very familiar with the LLVM code generator and so any advice on how to do this will be appreciated. I don't want to touch the code generator but if there is a way to tag the switch instruction and somehow get the jump table corresponding to that switch at run-time, that's all I need. If I can use intrinsics to achieve this, that would be great. Also is there a way to guarantee the generation of jump table for a switch instruction? My switch labels correspond to a continuous range of values [0..N].
One approach is not to use the switch instruction to lower my switch statement to LLVM. Instead, implement the jump table myself and use the indirectbr instruction. Then I have full control over the jump table. Is this feasible? How would I initialize my global array representing the jump table with the local labels in a function?
Another option is to use indirect tail calls and a table of function
pointers. E.g.
typedef uint32_t opcode;
typedef void dispatch_f(opcode *, long, long, long);
dispatch_f *jump_tab[256] = {
...
};
void add(opcode *pc, long r1, long r2) {
return jump_tab[*pc & 0xFF](pc + 1, r1 + r2, r2);
}
void sub(opcode *pc, long r1, long r2) {
return jump_tab[*pc & 0xFF](pc + 1, r1 - r2, r2);
}
This has the advantage that you can be sure that you get get a fixed
register assignment (which depends on the calling convention), which is
typically one of the most difficult things for the compiler to get right
for "interpreter-like" code in switches or with indirect branches.
To achieve what you want with this approach, you can just modify jump_tab.
Alternatively, you can swap out the jump table itself (e.g. to switch into
a tracing mode for a tracing jit).
-- Sean Silva