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[LangRef] Document the difference between <abi> and <pref> #147929

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nikic merged 4 commits into from
Jul 16, 2025
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[LangRef] Document the difference between <abi> and <pref> #147929

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[LangRef] Document the difference between `<abi>` and `<pref>`
tgross35 Jul 10, 2025
d2d1234
Be more specific about what abi does
tgross35 Jul 10, 2025
25d3736
Revert to existing list wording
tgross35 Jul 13, 2025
769a387
Describe where `pref` is used, rewrap to 80
tgross35 Jul 15, 2025
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13 changes: 8 additions & 5 deletions llvm/docs/LangRef.rst
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Expand Up @@ -3240,12 +3240,15 @@ as follows:
as :ref:`Non-Integral Pointer Type <nointptrtype>` s. The ``0``
address space cannot be specified as non-integral.

``<abi>`` provides a minimum allowed alignment for a type, and
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@nikic nikic Jul 10, 2025

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"minimum allowed alignment" isn't really accurate in that you can use lower alignments, they'll just be considered unaligned.

The ABI alignment is used in various places:

  • The default alignment for loads and stores when no explicit alignment is given in textual IR.
  • The alignment used to compute struct layout.
  • The alignment used to compute allocation sizes and thus getelementptr offsets.
  • The alignment below which accesses are considered underaligned.
  • ...

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@tgross35 tgross35 Jul 10, 2025

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I'll update to mention this. Are there any more specific cases of pref that should be mentioned?

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@efriedma-quic efriedma-quic Jul 14, 2025

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"pref" align is primarily used for allocas and global variables.

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@tgross35 tgross35 Jul 15, 2025

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Updated to mention this

``<pref>`` allows providing a more optimal alignment that should be used
when possible. ``<pref>`` is an optional value that must be greater than
or equal to ``<abi>``. If omitted, the preceding ``:`` should also be
omitted and ``<pref>`` will be equal to ``<abi>``.

Unless explicitly stated otherwise, on every specification that specifies
an alignment, the value of the alignment must be in the range [1,2^16)
and must be a power of two times the width of a byte.
On every specification that takes a ``<abi>:<pref>``, specifying the
``<pref>`` alignment is optional. If omitted, the preceding ``:``
should be omitted too and ``<pref>`` will be equal to ``<abi>``.

When constructing the data layout for a given target, LLVM starts with a
default set of specifications which are then (possibly) overridden by
Expand All @@ -3261,8 +3264,8 @@ specifications are given in this list:
- ``i8:8:8`` - i8 is 8-bit (byte) aligned as mandated
- ``i16:16:16`` - i16 is 16-bit aligned
- ``i32:32:32`` - i32 is 32-bit aligned
- ``i64:32:64`` - i64 has ABI alignment of 32-bits but preferred
alignment of 64-bits
- ``i64:32:64`` - i64 has a required alignment of 32-bits but should be
aligned to 64-bits if possible.
- ``f16:16:16`` - half is 16-bit aligned
- ``f32:32:32`` - float is 32-bit aligned
- ``f64:64:64`` - double is 64-bit aligned
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