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Introduce std.mem.readPackedInt and improve bitcasting of packed memory layouts #13221

Merged
merged 5 commits into from
Oct 29, 2022
Merged

Introduce std.mem.readPackedInt and improve bitcasting of packed memory layouts #13221

merged 5 commits into from
Oct 29, 2022

Commits on Oct 28, 2022

  1. std.mem: Add readPackedInt, writePackedInt, etc. 

    These utility functions allow reading from (stage2) packed memory at
runtime-known offsets.
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    topolarity committed Oct 28, 2022
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  2. stage2: Use mem.readPackedInt etc. for packed bitcasts 

    Packed memory has a well-defined layout that doesn't require
conversion from an integer to read from. Let's use it :-)

This change means that for bitcasting to/from a packed value that
is N layers deep, we no longer have to create N temporary big-ints
and perform N copies.

Other miscellaneous improvements:
  - Adds support for casting to packed enums and vectors
  - Fixes bitcasting to/from vectors outside of a packed struct
  - Adds a fast path for bitcasting <= u/i64
  - Fixes bug when bitcasting f80 which would clear following fields

This also changes the bitcast memory layout of exotic integers on
big-endian systems to match what's empirically observed on our targets.
Technically, this layout is not guaranteed by LLVM so we should probably
ban bitcasts that reveal these padding bits, but for now this is an
improvement.
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  3. Value: Add @intCast in writeToPackedMemory for 32-bit targets

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  4. std.mem: Skip read/writePackedInt test on WASM32/64

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  5. Enable bitcast test now that ziglang#13214 is resolved.

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