Implement loop peeling.

[ptersilie]

This implements a basic version of loop peeling. Note, that additional optimisations based on loop peeling, like common subexpression eliminiation, are not included in this PR. Those will be implemented in subsequent PRs.

[ltratt]

On a fresh clone (and even if I merge in master), I see a different error:

thread '<unnamed>' panicked at ykrt/src/compile/jitc_yk/jit_ir/mod.rs:333:19:
index out of bounds: the len is 67 but the index is 69
stack backtrace:
   0: rust_begin_unwind
             at /rustc/5e1440ae514d98ddfcbf1607acb64d41e07ef616/library/std/src/panicking.rs:681:5
   1: core::panicking::panic_fmt
             at /rustc/5e1440ae514d98ddfcbf1607acb64d41e07ef616/library/core/src/panicking.rs:75:14
   2: core::panicking::panic_bounds_check
             at /rustc/5e1440ae514d98ddfcbf1607acb64d41e07ef616/library/core/src/panicking.rs:273:5
   3: <usize as core::slice::index::SliceIndex<[T]>>::index
             at /rustc/5e1440ae514d98ddfcbf1607acb64d41e07ef616/library/core/src/slice/index.rs:274:10
   4: core::slice::index::<impl core::ops::index::Index<I> for [T]>::index
             at /rustc/5e1440ae514d98ddfcbf1607acb64d41e07ef616/library/core/src/slice/index.rs:16:9
   5: <alloc::vec::Vec<T,A> as core::ops::index::Index<I>>::index
             at /rustc/5e1440ae514d98ddfcbf1607acb64d41e07ef616/library/alloc/src/vec/mod.rs:3346:9
   6: ykrt::compile::jitc_yk::jit_ir::Module::inst_raw
             at /vol/extra/ltratt/yk/ykrt/src/compile/jitc_yk/jit_ir/mod.rs:333:19
   7: ykrt::compile::jitc_yk::jit_ir::PackedOperand::unpack
             at /vol/extra/ltratt/yk/ykrt/src/compile/jitc_yk/jit_ir/mod.rs:1104:23
   8: ykrt::compile::jitc_yk::jit_ir::Inst::map_operand_locals
             at /vol/extra/ltratt/yk/ykrt/src/compile/jitc_yk/jit_ir/mod.rs:1588:21
   9: ykrt::compile::jitc_yk::jit_ir::dead_code::<impl ykrt::compile::jitc_yk::jit_ir::Module>::dead_code_elimination
             at /vol/extra/ltratt/yk/ykrt/src/compile/jitc_yk/jit_ir/dead_code.rs:20:17
  10: ykrt::compile::jitc_yk::opt::Opt::opt
             at /vol/extra/ltratt/yk/ykrt/src/compile/jitc_yk/opt/mod.rs:426:9
  11: ykrt::compile::jitc_yk::opt::opt
             at /vol/extra/ltratt/yk/ykrt/src/compile/jitc_yk/opt/mod.rs:567:5

[ltratt]

We will need at least one (and preferably more) C tests for this: they don't necessarily have to be super-super complex, but we should at least check that jit-post-opt has peeled the loop!

[ltratt]

Right, so just the missing C tests left I think.

[ptersilie]

I've just added one simple test. I'm currently trying to make a C tests where we can see constant forwarding into the body, but I haven't had any luck so far.

[ltratt]

I'm currently trying to make a C tests where we can see constant forwarding into the body, but I haven't had any luck so far.

I think yk_promote will do what you want.

[ptersilie]

I think yk_promote will do what you want.

I tried all the promote tests. But it seems because we are using NOOPT_VAL they are always loaded from the stack and thus not passed into the body.

[ltratt]

Just noticed: this should be (&Module, InstIdx).

[ptersilie]

7aeae43

[ltratt]

This is almost certainly unsafe (similarly for TraceHeaderEnd below): it means that calling this method permanently mutates the global notion of TraceHeaderStart. If you call this method twice with a TraceHeaderStart, weird things are likely to happen.

IMHO: either users are forbidden from calling this function with TraceHeaderStart | TraceHeaderEnd or we work out some way of not mutating things globally.

[ltratt]

Please squash.

[ptersilie]

Squashed and rebased.