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From: "deller at gmx dot de" <gcc-bugzilla@gcc.gnu.org>
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Subject: [Bug tree-optimization/102162] Byte-wise access optimized away at
 -O1 and above
Date: Wed, 01 Sep 2021 21:25:56 +0000
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https://gcc.gnu.org/bugzilla/show_bug.cgi?id=3D102162

--- Comment #8 from deller at gmx dot de ---
On 9/1/21 11:19 PM, dave.anglin at bell dot net wrote:
>> I think the problem with your testcase is, that the compiler doesn't kno=
w the
>> alignment of the parameter "p" in your f_unaligned() function.
>> So it will generate byte-accesses.
> I think it's the type rather than the alignment.=C2=A0 If type is char, o=
ne gets
> byte accesses.=C2=A0 If type is short, one gets 16-bit accesses.
>
> The alignment is being ignored.

You are right.
It's even worse!

short evil;
int f_unaligned2(void)
{ return get_unaligned(&evil); }

gives:
00000000 <f_unaligned2>:
    0:   2b 60 00 00     addil L%0,dp,r1
    4:   44 3c 00 00     ldh 0(r1),ret0
    8:   e8 40 c0 00     bv r0(rp)
    c:   d3 9c 1f f0     extrw,s ret0,31,16,ret0

The "ldh" loads only the first two bytes, and extends it into the upper 32b=
its
with "extrw,s".
So, only 16bits instead of 32bits are loaded from the address where "evil"
is...=