Stephan Renatus 5a1ed9c7fa wasm: replace unused functions by stub (#3206) ...

We're in this situation: performing dead code analysis on wasm isn't too
hard, but it requires a representation of all wasm instructions: we'd need
to be able to parse the "runtime" wasm bits, i.e., what's built using llvm
from C code. When building upon that wasm module, we process the function
bodies uninterpreted -- they are all just `[]byte` to us.

This restriction lets us get by without implementing all the wasm
instructions -- we only write what we use, and read a bare minimum to work
as outlined above.

To still be able to remove dead code, this change employs a trick: at build
time, when the aforementioned runtime wasm module is compiled, we're calling
wasm-opt on it to extract its call graph. We'll use that, together with the
functions actually planned in our wasm compiler (using the subset of
instructions that we understand), to remove all unused functions from the
name section, and replace their function bodies with `unreachable`.

We cannot really remove them, since that would require reindexing all
functions; and we cannot do that without replacing the function indices at
their call sites in the "runtime" wasm module.

Another restriction to the impact of this approach is call_indirect: We
need to keep every function that's referenced in the table -- we don't know
which function might be calling them indirectly. In a follow-up, we could
record that information and use it to further reduce the code size: we know
that if none of the regex-related builtins are used, we could also stub out
the re2-related functions.

Signed-off-by: Stephan Renatus <stephan.renatus@gmail.com>

2021-03-11 10:25:16 +01:00

..

gen-wasm-callgraph.sh

wasm: replace unused functions by stub (#3206)

2021-03-11 10:25:16 +01:00