[llvm-dev] Side-channel resistant values

David Zarzycki via llvm-dev
2019-09-14T05:19:26Z

I’m struggling to find cases where __builtin_unpredictable() works at all. Even
if we ignore cmp/br into switch conversion, it still doesn’t work:

int test_cmov(int left, int right, int *alt) {
	return __builtin_unpredictable(left < right) ? *alt : 999;
}

Should generate:

test_cmov:
	movl	$999, %eax
	cmpl	%esi, %edi
	cmovll	(%rdx), %eax
	retq

But currently generates:

test_cmov:
	movl	$999, %eax
	cmpl	%esi, %edi
	jge	.LBB0_2
	movl	(%rdx), %eax
.LBB0_2:
	retq



> On Sep 14, 2019, at 12:18 AM, Sanjay Patel <spatel at
rotateright.com> wrote:
>
> I'm not sure if this is the entire problem, but SimplifyCFG loses the
'unpredictable' metadata when it converts a set of cmp/br into a switch:
> https://godbolt.org/z/neLzN3
>
> Filed here:
> https://bugs.llvm.org/show_bug.cgi?id=43313
>
> On Fri, Sep 13, 2019 at 4:02 AM David Zarzycki via llvm-dev <llvm-dev at
lists.llvm.org> wrote:
>
>
>> On Sep 13, 2019, at 10:45 AM, Chandler Carruth <chandlerc at
gmail.com> wrote:
>>
>> On Fri, Sep 13, 2019 at 1:33 AM David Zarzycki via llvm-dev
<llvm-dev at lists.llvm.org> wrote:
>> Hi Chandler,
>>
>> The data-invariant feature sounds great but what about the general
case? When performance tuning code, people sometimes need the ability to
reliably generate CMOV, and right now the best advice is either “use inline
assembly” or “keep refactoring until CMOV is emited” (and hope that future
compilers continue to generate CMOV).
>>
>> Given that a patch already exists to reliably generate CMOV, are there
any good arguments against adding the feature?
>>
>> For *performance* tuning, the builtin that Hal mentioned is IMO the
correct design.
>>
>> Is there some reason why it doesn't work?
>
> I wasn’t aware of __builtin_unpredictable() until now and I haven’t
debugged why it doesn’t work, but here are a couple examples, one using the
ternary operator and one using a switch statement:
>
> https://godbolt.org/z/S46I_q
>
> Dave
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