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std: Minimize size of panicking on wasm #49488

Merged
merged 4 commits into from
Apr 17, 2018
Merged

std: Minimize size of panicking on wasm #49488

merged 4 commits into from
Apr 17, 2018

Commits on Apr 13, 2018

  1. std: Minimize size of panicking on wasm 

    This commit applies a few code size optimizations for the wasm target to
the standard library, namely around panics. We notably know that in most
configurations it's impossible for us to print anything in
wasm32-unknown-unknown so we can skip larger portions of panicking that
are otherwise simply informative. This allows us to get quite a nice
size reduction.

Finally we can also tweak where the allocation happens for the
`Box<Any>` that we panic with. By only allocating once unwinding starts
we can reduce the size of a panicking wasm module from 44k to 350 bytes.
    @alexcrichton
    alexcrichton committed Apr 13, 2018
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  2. Reduce the size of panics in RawVec 

    Create one canonical location which panics with "capacity overflow" instead of
having many. This reduces the size of a `panic!("{}", 1)` binary on wasm from
34k to 17k.
    @alexcrichton
    alexcrichton committed Apr 13, 2018
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  3. core: Remove an implicit panic from Formatter::pad 

    The expression `&s[..i]` in general can panic if `i` is out of bounds or not on
a character boundary for a string, and this caused the codegen for
`Formatter::pad` to be a bit larger than it otherwise needed to be. This commit
replaces this with `s.get(..i).unwrap_or(&s)` which while having different
behavior if `i` is out of bounds has a much smaller code footprint and otherwise
avoids the need for `unsafe` code.
    @alexcrichton
    alexcrichton committed Apr 13, 2018
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  4. std: Avoid allocating panic message unless needed 

    This commit removes allocation of the panic message in instances like
`panic!("foo: {}", "bar")` if we don't actually end up needing the message. We
don't need it in the case of wasm32 right now, and in general it's not needed
for panic=abort instances that use the default panic hook.

For now this commit only solves the wasm use case where with LTO the allocation
is entirely removed, but the panic=abort use case can be implemented at a later
date if needed.
    @alexcrichton
    alexcrichton committed Apr 13, 2018
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