I found that when I enable ThinLTO, a miscompilation triggers that had
been hidden all the time previously. The bug appears to happen as
follows:
1. TinyGo generates a function with a runtime.trackPointer call, but
without an alloca (or the alloca gets optimized away).
2. LLVM sees that no alloca needs to be kept alive across the
runtime.trackPointer call, and therefore it adds the 'tail' flag.
One of the effects of this flag is that it makes it undefined
behavior to keep allocas alive across the call (which is still safe
at that point).
3. The GC lowering pass adds a stack slot alloca and converts
runtime.trackPointer calls into alloca stores.
The last step triggers the bug: the compiler inserts an alloca where
there was none before but that's not valid as long as the 'tail' flag is
present.
This patch fixes the bug in a somewhat dirty way, by always creating a
dummy alloca so that LLVM won't do the optimization in step 2 (and
possibly other optimizations that rely on there being no alloca
instruction).
This implements the block-based GC as a partially precise GC. This means
that for most heap allocations it is known which words contain a pointer
and which don't. This should in theory make the GC faster (because it
can skip non-pointer object) and have fewer false positives in a GC
cycle. It does however use a bit more RAM to store the layout of each
object.
Right now this GC seems to be slower than the conservative GC, but
should be less likely to run out of memory as a result of false
positives.
Most of the code of the conservative GC can be reused for the precise
GC. So before adding precise GC support, this commit just moves code
around to make the next commit cleaner. It is a non-functional change.
Proof: https://godbolt.org/z/as4EM3713
Essentially, this means that there are objects on arm64 that have a
16-byte alignment and so we have to respect that when we allocate things
on the heap.
This function provides a mechanism to watch for changes to the GODEBUG
environment variable. For now, we'll not implement it. It might be
useful in the future, when it can always be added.
wasmtime by default will assume the subcommand is "run" vs one of its
others, but being explicit helps clarify the actual command invoked.
For example, we pass similar looking args to wasmtime and also wasi.
Signed-off-by: Adrian Cole <adrian@tetrate.io>
We don't support these yet so let's just put them in a central location.
Once these functions are supported we can think about how to structure
the code again.
ThinLTO results in a small code size reduction, which is nice
(especially on these very small chips). It also brings us one step
closer to using ThinLTO everywhere.
LLVM wasn't aware that runtime.stackChainStart must be kept live and
can't be optimized away. With this hack, it is forced to consider
stackChainStart live at the time of the stack scan.
This fixes the corruption in https://github.com/tinygo-org/tinygo/issues/3277
This reverts commit 0b3a7280fa and updates
the documentation a little bit to explain the purpose of -gc=none. (I'm
thinking about the attiny10 by the way where defaulting to -gc=none
makes sense).
