Use relative call instructions between wasm functions (#3275)

* Use relative `call` instructions between wasm functions

This commit is a relatively major change to the way that Wasmtime
generates code for Wasm modules and how functions call each other.
Prior to this commit all function calls between functions, even if they
were defined in the same module, were done indirectly through a
register. To implement this the backend would emit an absolute 8-byte
relocation near all function calls, load that address into a register,
and then call it. While this technique is simple to implement and easy
to get right, it has two primary downsides associated with it:

* Function calls are always indirect which means they are more difficult
  to predict, resulting in worse performance.

* Generating a relocation-per-function call requires expensive
  relocation resolution at module-load time, which can be a large
  contributing factor to how long it takes to load a precompiled module.

To fix these issues, while also somewhat compromising on the previously
simple implementation technique, this commit switches wasm calls within
a module to using the `colocated` flag enabled in Cranelift-speak, which
basically means that a relative call instruction is used with a
relocation that's resolved relative to the pc of the call instruction
itself.

When switching the `colocated` flag to `true` this commit is also then
able to move much of the relocation resolution from `wasmtime_jit::link`
into `wasmtime_cranelift::obj` during object-construction time. This
frontloads all relocation work which means that there's actually no
relocations related to function calls in the final image, solving both
of our points above.

The main gotcha in implementing this technique is that there are
hardware limitations to relative function calls which mean we can't
simply blindly use them. AArch64, for example, can only go +/- 64 MB
from the `bl` instruction to the target, which means that if the
function we're calling is a greater distance away then we would fail to
resolve that relocation. On x86_64 the limits are +/- 2GB which are much
larger, but theoretically still feasible to hit. Consequently the main
increase in implementation complexity is fixing this issue.

This issue is actually already present in Cranelift itself, and is
internally one of the invariants handled by the `MachBuffer` type. When
generating a function relative jumps between basic blocks have similar
restrictions. This commit adds new methods for the `MachBackend` trait
and updates the implementation of `MachBuffer` to account for all these
new branches. Specifically the changes to `MachBuffer` are:

* For AAarch64 the `LabelUse::Branch26` value now supports veneers, and
  AArch64 calls use this to resolve relocations.

* The `emit_island` function has been rewritten internally to handle
  some cases which previously didn't come up before, such as:

  * When emitting an island the deadline is now recalculated, where
    previously it was always set to infinitely in the future. This was ok
    prior since only a `Branch19` supported veneers and once it was
    promoted no veneers were supported, so without multiple layers of
    promotion the lack of a new deadline was ok.

  * When emitting an island all pending fixups had veneers forced if
    their branch target wasn't known yet. This was generally ok for
    19-bit fixups since the only kind getting a veneer was a 19-bit
    fixup, but with mixed kinds it's a bit odd to force veneers for a
    26-bit fixup just because a nearby 19-bit fixup needed a veneer.
    Instead fixups are now re-enqueued unless they're known to be
    out-of-bounds. This may run the risk of generating more islands for
    19-bit branches but it should also reduce the number of islands for
    between-function calls.

  * Otherwise the internal logic was tweaked to ideally be a bit more
    simple, but that's a pretty subjective criteria in compilers...

I've added some simple testing of this for now. A synthetic compiler
option was create to simply add padded 0s between functions and test
cases implement various forms of calls that at least need veneers. A
test is also included for x86_64, but it is unfortunately pretty slow
because it requires generating 2GB of output. I'm hoping for now it's
not too bad, but we can disable the test if it's prohibitive and
otherwise just comment the necessary portions to be sure to run the
ignored test if these parts of the code have changed.

The final end-result of this commit is that for a large module I'm
working with the number of relocations dropped to zero, meaning that
nothing actually needs to be done to the text section when it's loaded
into memory (yay!). I haven't run final benchmarks yet but this is the
last remaining source of significant slowdown when loading modules,
after I land a number of other PRs both active and ones that I only have
locally for now.

* Fix arm32

* Review comments

crates/jit/src/link.rs

@@ -1,11 +1,10 @@
 //! Linking for JIT-compiled code.
 
 use object::read::{Object, Relocation, RelocationTarget};
-use object::{elf, File, NativeEndian as NE, ObjectSymbol, RelocationEncoding, RelocationKind};
+use object::{File, NativeEndian as NE, ObjectSymbol, RelocationEncoding, RelocationKind};
 use std::convert::TryFrom;
 use wasmtime_runtime::libcalls;
 
-type U32 = object::U32Bytes<NE>;
 type I32 = object::I32Bytes<NE>;
 type U64 = object::U64Bytes<NE>;
 
@@ -36,11 +35,10 @@ pub fn apply_reloc(obj: &File, code: &mut [u8], offset: u64, r: Relocation) {
                 }
             }
         }
-        _ => panic!("unexpected relocation target"),
+        _ => panic!("unexpected relocation target: not a symbol"),
     };
 
     match (r.kind(), r.encoding(), r.size()) {
-        #[cfg(target_pointer_width = "64")]
         (RelocationKind::Absolute, RelocationEncoding::Generic, 64) => {
             let reloc_address = reloc_address::<U64>(code, offset);
             let reloc_abs = (target_func_address as u64)
@@ -48,63 +46,17 @@ pub fn apply_reloc(obj: &File, code: &mut [u8], offset: u64, r: Relocation) {
                 .unwrap();
             reloc_address.set(NE, reloc_abs);
         }
-        #[cfg(target_pointer_width = "32")]
-        (RelocationKind::Relative, RelocationEncoding::Generic, 32) => {
-            let reloc_address = reloc_address::<U32>(code, offset);
-            let reloc_delta_u32 = (target_func_address as u32)
-                .wrapping_sub(reloc_address as *const _ as u32)
-                .checked_add(r.addend() as u32)
-                .unwrap();
-            reloc_address.set(NE, reloc_delta_u32);
-        }
-        #[cfg(target_pointer_width = "32")]
-        (RelocationKind::Relative, RelocationEncoding::X86Branch, 32) => {
-            let reloc_address = reloc_address::<U32>(code, offset);
-            let reloc_delta_u32 = (target_func_address as u32)
-                .wrapping_sub(reloc_address as *const _ as u32)
-                .wrapping_add(r.addend() as u32);
-            reloc_address.set(NE, reloc_delta_u32);
-        }
-        #[cfg(target_pointer_width = "64")]
+
+        // FIXME(#3009) after the old backend is removed this won't ever show up
+        // again so it can be removed.
         (RelocationKind::Relative, RelocationEncoding::Generic, 32) => {
             let reloc_address = reloc_address::<I32>(code, offset);
-            let reloc_delta_i64 = (target_func_address as i64)
-                .wrapping_sub(reloc_address as *const _ as i64)
-                .wrapping_add(r.addend());
-            // TODO implement far calls mode in x64 new backend.
-            reloc_address.set(
-                NE,
-                i32::try_from(reloc_delta_i64).expect("relocation too large to fit in i32"),
-            );
-        }
-        #[cfg(target_pointer_width = "64")]
-        (RelocationKind::Relative, RelocationEncoding::S390xDbl, 32) => {
-            let reloc_address = reloc_address::<I32>(code, offset);
-            let reloc_delta_i64 = (target_func_address as i64)
-                .wrapping_sub(reloc_address as *const _ as i64)
+            let val = (target_func_address as i64)
                 .wrapping_add(r.addend())
-                >> 1;
-            reloc_address.set(
-                NE,
-                i32::try_from(reloc_delta_i64).expect("relocation too large to fit in i32"),
-            );
-        }
-        (RelocationKind::Elf(elf::R_AARCH64_CALL26), RelocationEncoding::Generic, 32) => {
-            let reloc_address = reloc_address::<U32>(code, offset);
-            let reloc_delta = (target_func_address as u64).wrapping_sub(r.addend() as u64);
-            // TODO: come up with a PLT-like solution for longer calls. We can't extend the
-            // code segment at this point, but we could conservatively allocate space at the
-            // end of the function during codegen, a fixed amount per call, to allow for
-            // potential branch islands.
-            assert!((reloc_delta as i64) < (1 << 27));
-            assert!((reloc_delta as i64) >= -(1 << 27));
-            let reloc_delta = reloc_delta as u32;
-            let reloc_delta = reloc_delta.wrapping_add(r.addend() as u32);
-            let delta_bits = reloc_delta >> 2;
-            let insn = reloc_address.get(NE);
-            let new_insn = (insn & 0xfc00_0000) | (delta_bits & 0x03ff_ffff);
-            reloc_address.set(NE, new_insn);
+                .wrapping_sub(reloc_address as *const _ as i64);
+            reloc_address.set(NE, i32::try_from(val).expect("relocation out-of-bounds"));
         }
+
         other => panic!("unsupported reloc kind: {:?}", other),
     }
 }