Wrong alignment on memory loads with non-default alignment

[JohanEngelen]

I was very surprised to find this bug, so I hope I am wrong.

Testcase (https://d.godbolt.org/z/aYoM9odTo):

struct S {
    byte i;
    align(1) long data;
}
long foo_s( S* d) { return d.data; }

Is compiled to:

%example.S = type <{ i8, i64 }>

define i64 @_D7example5foo_sFPSQr1SZl(%example.S* %d_arg) #0 !dbg !4 {
  %d = alloca %example.S*, align 8                ; [#uses = 2, size/byte = 8]
  store %example.S* %d_arg, %example.S** %d, align 8, !dbg !7 ; [debug line = app/example.d:6:6]
  %1 = load %example.S*, %example.S** %d, align 8, !dbg !8 ; [#uses = 1] [debug line = app/example.d:6:21]
  %2 = getelementptr inbounds %example.S, %example.S* %1, i32 0, i32 1 ; [#uses = 2, type = i64*]
  %3 = load i64, i64* %2, align 8, !dbg !8        ; [#uses = 0] [debug line = app/example.d:6:21]
  %4 = load i64, i64* %2, align 8, !dbg !8        ; [#uses = 1] [debug line = app/example.d:6:21]
  ret i64 %4, !dbg !8                             ; [debug line = app/example.d:6:21]
}

Note the align 8 on loading d.data, whereas it should be align 1. This works on x86 but may break on CPU architecturers where some instructrions do not support unaligned loads.

[p0nce]

I don't plan on relying on this :) I think I'll just use void* and mark the functions as @system, will keep compat with copy-pasted code, and some of the other intrinsics are @system as per their signature

[JohanEngelen]

This issue is probably pervasive throughout LDC.

align(1) float float1;
extern(C) auto fooglobalglobal() {
    return float1; // also does not create IR with align(1)
}

For the struct field, the frontend .alignof property is also wrong (https://issues.dlang.org/show_bug.cgi?id=23649)

struct S {
    byte i;
    align(1) long data;
}

void foo( S* d) {
    pragma(msg, d.data.alignof); // prints 8UL
}

[JohanEngelen]

Example lit test:

// RUN: %ldc -output-ll -of=%t.ll %s && FileCheck %s < %t.ll

struct S {
    byte i;
    align(1) long data;
}

// CHECK-LABEL: define{{.*}} @foofoofoo
extern(C) long foofoofoo(S *d) {
    // CHECK: load i64, {{i64\*|ptr}} %2, align 1
    return d.data;
// CHECK-LABEL: ret i64
}

align(1) float float1;
// CHECK-LABEL: define{{.*}} @fooglobalglobal
extern(C) auto fooglobalglobal() {
    // CHECK: load float, {{float\*|ptr}} @_D18align_loads_gh42366float1f, align 1
    return float1;
// CHECK-LABEL: ret float
}