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llvm: fix lowering of non-byte-aligned field pointers

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* When a field starts at some bit offset within a byte you need to load
   starting from that byte and shift, not starting from the next byte,
   so a rounded-down divide is required here, not a rounded-up one.
 * Remove paragraph from doc that no longer relates to anything.

Closes #12363
This commit is contained in:
Jacob Young 2022-10-15 03:31:17 -04:00 • committed by Andrew Kelley
parent c7f9833238
commit f9192adaba
2 changed files with 1 additions and 6 deletions
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5
doc/langref.html.in
View file

@ -3341,7 +3341,6 @@ fn doTheTest() !void {
Zig allows the address to be taken of a non-byte-aligned field:
</p>
{#code_begin|test|pointer_to_non-byte_aligned_field#}
{#backend_stage1#}
const std = @import("std");
const expect = std.testing.expect;
@ -3398,7 +3397,6 @@ fn bar(x: *const u3) u3 {
Pointers to non-ABI-aligned fields share the same address as the other fields within their host integer:
</p>
{#code_begin|test|packed_struct_field_addrs#}
{#backend_stage1#}
const std = @import("std");
const expect = std.testing.expect;
@ -3463,9 +3461,6 @@ test "overaligned pointer to packed struct" {
try expect(ptr_to_b.* == 2);
}
{#code_end#}
<p>When this bug is fixed, the above test in the documentation will unexpectedly pass, which will
cause the test suite to fail, notifying the bug fixer to update these docs.
</p>
<p>
It's also possible to set alignment of struct fields:
</p>

2
src/codegen/llvm.zig
View file

@ -3943,7 +3943,7 @@ pub const DeclGen = struct {
}
break :b b;
};
const byte_offset = llvm_usize.constInt((prev_bits + 7) / 8, .False);
const byte_offset = llvm_usize.constInt(prev_bits / 8, .False);
const field_addr = base_addr.constAdd(byte_offset);
bitcast_needed = false;
const final_llvm_ty = (try dg.lowerType(ptr_child_ty)).pointerType(0);