From f9192adaba0eb344ed12aad9c675cd73b740d2a2 Mon Sep 17 00:00:00 2001 From: Jacob Young Date: Sat, 15 Oct 2022 03:31:17 -0400 Subject: [PATCH] llvm: fix lowering of non-byte-aligned field pointers * 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 --- doc/langref.html.in | 5 ----- src/codegen/llvm.zig | 2 +- 2 files changed, 1 insertion(+), 6 deletions(-) diff --git a/doc/langref.html.in b/doc/langref.html.in index 872c305252..cb397eb446 100644 --- a/doc/langref.html.in +++ b/doc/langref.html.in @@ -3341,7 +3341,6 @@ fn doTheTest() !void { Zig allows the address to be taken of a non-byte-aligned field:

{#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:

{#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#} -

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. -

It's also possible to set alignment of struct fields:

diff --git a/src/codegen/llvm.zig b/src/codegen/llvm.zig index 3a02cd630b..cc18109270 100644 --- a/src/codegen/llvm.zig +++ b/src/codegen/llvm.zig @@ -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);