[llvm-dev] Aarch64: unaligned access despite -mstrict-align

[Sjoerd Meijer via llvm-dev]

Sorry, quick message to ignore what I wrote before, I got myself confused (probably you too),
With a recent trunk build I get this:

f:
        adrp    x8, g
        ldr     x8, [x8, :lo12:g]
        mov     w2, #16
        mov     x1, x0
        mov     x0, x8
        b       memcmp

This looks more correct, and I need to look a bit more into this (and how clang 10.0.0 behaves).


________________________________
From: llvm-dev <llvm-dev-bounces at lists.llvm.org> on behalf of Sjoerd Meijer via llvm-dev <llvm-dev at lists.llvm.org>
Sent: 01 June 2020 16:32
To: llvm-dev at lists.llvm.org <llvm-dev at lists.llvm.org>; Jerome Forissier <jerome at forissier.org>
Cc: op-tee at lists.trustedfirmware.org <op-tee at lists.trustedfirmware.org>
Subject: Re: [llvm-dev] Aarch64: unaligned access despite -mstrict-align

Hi Jerome,

> Note the 'ldr x9, [x0]'. At this point there is no guarantee that x0 is a multiple of 8, so why is Clang generating this code?

I think the point is that it can assume it is 8 byte aligned, so the question is why it isn't.  I guess that requires looking into how memory is allocated to what C is pointing to, or if some type punning caused passing an address that is not properly aligned.

Cheers,
Sjoerd.
________________________________
From: llvm-dev <llvm-dev-bounces at lists.llvm.org> on behalf of Jerome Forissier via llvm-dev <llvm-dev at lists.llvm.org>
Sent: 01 June 2020 15:26
To: llvm-dev at lists.llvm.org <llvm-dev at lists.llvm.org>
Cc: op-tee at lists.trustedfirmware.org <op-tee at lists.trustedfirmware.org>
Subject: [llvm-dev] Aarch64: unaligned access despite -mstrict-align

Hi,

I experienced a crash in code compiled with Clang 10.0.0 due to a
misaligned 64-bit data access. The (ARMv8) CPU is configured with SCTL.A
== 1 (alignment check enable). With SCTLR.A == 0 the code runs as expected.

After some investigation I came up with the following reproducer:

---8<-------8<-------8<-------8<-------8<-------8<-------8<-------
$ cat test.c
extern char *g;
int memcmp(const void *s1, const void *s2, unsigned long n);

int f(void *c)
{
        return memcmp(g, c, 16);
}
$ clang --target=aarch64-linux-gnu -Os -mstrict-align -S test.c
$ cat test.s
        .text
        .file   "test.c"
        .globl  f                       // -- Begin function f
        .p2align        2
        .type   f, at function
f:                                      // @f
// %bb.0:
        adrp    x8, g
        ldr     x10, [x8, :lo12:g]
        ldr     x9, [x0]
        ldr     x8, [x10]
        rev     x9, x9
        rev     x8, x8
        cmp     x8, x9
        b.ne    .LBB0_3
// %bb.1:
        ldr     x8, [x10, #8]
        ldr     x9, [x0, #8]
        rev     x8, x8
        rev     x9, x9
        cmp     x8, x9
        b.ne    .LBB0_3
// %bb.2:
        mov     w0, wzr
        ret
.LBB0_3:
        cmp     x8, x9
        mov     w8, #-1
        cneg    w0, w8, hs
        ret
.Lfunc_end0:
        .size   f, .Lfunc_end0-f
                                        // -- End function
        .ident  "clang version 10.0.0-4ubuntu1 "
        .section        ".note.GNU-stack","", at progbits
        .addrsig
---8<-------8<-------8<-------8<-------8<-------8<-------8<-------

Note the 'ldr x9, [x0]'. At this point there is no guarantee that x0 is
a multiple of 8, so why is Clang generating this code?

Thanks,
--
Jerome
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