[LangRef] Require i8s to be naturally aligned

[jsilvanus]

Summary

It is widely assumed that i8 is naturally aligned (i8:8),
and that hence i8s can be used to access arbitrary bytes.

As discussed in https://discourse.llvm.org/t/status-of-overaligned-i8,
this patch makes this assumption explicit, by documenting it in
the LangRef, and enforcing it when parsing a data layout string.

Historically, there have been data layouts that violate this requirement,
notably the old DXIL data layout that aligns i8 to 32 bits.

A previous patch (df1a74a) enabled importing modules with invalid data layouts
using override callbacks.
Users who wish to continue importing modules with overaligned i8s (e.g. DXIL)
thus need to provide a data layout override callback that fixes the
data layout, at minimum by setting natural alignment for i8.

Any further adjustments to the module (e.g. adding padding bytes if necessary)
need to be done after module import. In the case of DXIL, this should not be
necessary, because i8 usage in DXIL is very limited and its alignment actually
does not matter, see
https://github.com/microsoft/DirectXShaderCompiler/blob/main/docs/DXIL.rst#primitive-types

[lebedev.ri]

Yay!

[nikic]

LGTM

llvm/lib/IR/DataLayout.cpp

409

I'd suggest adding an AlignType == INTEGER_ALIGN && here. Probably doesn't matter in practice, but that limits it more explicitly to i8.

[jsilvanus]

Restrict alignment check to integer types as suggested by review.

[jsilvanus]

Thanks for the review, I've updated the patch accordingly.

llvm/lib/IR/DataLayout.cpp

409

Thanks for the suggestion, I added the restriction.
I've seen some recent discussions about fp8 types, so it may not be that irrelevant after all.

llvm/docs/LangRef.rst

``p[n]:<size>:<abi>[:<pref>][:<idx>]``
     specified, the default index size is equal to the pointer size. All sizes
     are in bits. The address space, ``n``, is optional, and if not specified,
     denotes the default address space 0. The value of ``n`` must be
     in the range [1,2^23).
 ``i<size>:<abi>[:<pref>]``
     This specifies the alignment for an integer type of a given bit
     ``<size>``. The value of ``<size>`` must be in the range [1,2^23).
     ``<pref>`` is optional and defaults to ``<abi>``.
+    For ``i8``, the ``<abi>`` value must equal 8,
+    that is, ``i8`` must be naturally aligned.
 ``v<size>:<abi>[:<pref>]``
     This specifies the alignment for a vector type of a given bit
     ``<size>``. The value of ``<size>`` must be in the range [1,2^23).
     ``<pref>`` is optional and defaults to ``<abi>``.
 ``f<size>:<abi>[:<pref>]``
     This specifies the alignment for a floating-point type of a given bit
     ``<size>``. Only values of ``<size>`` that are supported by the target
     will work. 32 (float) and 64 (double) are supported on all targets; 80

 specifications are given in this list:
 
 -  ``e`` - little endian
 -  ``p:64:64:64`` - 64-bit pointers with 64-bit alignment.
 -  ``p[n]:64:64:64`` - Other address spaces are assumed to be the
    same as the default address space.
 -  ``S0`` - natural stack alignment is unspecified
 -  ``i1:8:8`` - i1 is 8-bit (byte) aligned
--  ``i8:8:8`` - i8 is 8-bit (byte) aligned
+-  ``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
 -  ``f16:16:16`` - half is 16-bit aligned
 -  ``f32:32:32`` - float is 32-bit aligned
 -  ``f64:64:64`` - double is 64-bit aligned
 -  ``f128:128:128`` - quad is 128-bit aligned

llvm/lib/IR/DataLayout.cpp

case 'a': {
       if (AlignType != AGGREGATE_ALIGN && !ABIAlign)
         return reportError(
             "ABI alignment specification must be >0 for non-aggregate types");
 
       if (!isUInt<16>(ABIAlign))
         return reportError("Invalid ABI alignment, must be a 16bit integer");
       if (ABIAlign != 0 && !isPowerOf2_64(ABIAlign))
         return reportError("Invalid ABI alignment, must be a power of 2");
+      if (AlignType == INTEGER_ALIGN && Size == 8 && ABIAlign != 1)
+        return reportError(
+            "Invalid ABI alignment, i8 must be naturally aligned");
 
       // Preferred alignment.
       unsigned PrefAlign = ABIAlign;
       if (!Rest.empty()) {
         if (Error Err = ::split(Rest, ':', Split))
           return Err;
         if (Error Err = getIntInBytes(Tok, PrefAlign))
           return Err;

llvm/test/Assembler/datalayout-invalid-i8-alignment.ll

+; RUN: not llvm-as %s 2>&1 | FileCheck %s
+
+; CHECK: error: Invalid ABI alignment, i8 must be naturally aligned
+
+target datalayout = "i8:16"