Re: [PATCH] MdePkg/DebugLib; swap if conditions in ASSERT_[EFI|RETURN]_ERROR

[Ard Biesheuvel <ard.biesheuvel@linaro.org>]

On 7 December 2017 at 19:49, Kinney, Michael D
<michael.d.kinney@intel.com> wrote:
> Ard,
>
> I do not disagree with your logic.
>
> The current algorithm is based on data from a long
> time ago using what are now very old tool chains.
>

With LTO?

> I will do some experiments on the currently supported
> toolchains to see if the optimization is the same either
> way.
>

Thank you.

> I think the change you are suggesting is to improve
> performance for optimization disabled builds by removing
> an extra call.  Is that correct?
>

Well, for DEBUG builds, yes. But given that the function call cannot
be optimized away (on non-LTO builds), it affects optimized builds as
well.

-- 
Ard.


>> -----Original Message-----
>> From: Ard Biesheuvel [mailto:ard.biesheuvel@linaro.org]
>> Sent: Thursday, December 7, 2017 9:43 AM
>> To: Kinney, Michael D <michael.d.kinney@intel.com>
>> Cc: Alexei Fedorov <Alexei.Fedorov@arm.com>; edk2-
>> devel@lists.01.org; Gao, Liming <liming.gao@intel.com>;
>> Leif Lindholm <leif.lindholm@linaro.org>
>> Subject: Re: [edk2] [PATCH] MdePkg/DebugLib; swap if
>> conditions in ASSERT_[EFI|RETURN]_ERROR
>>
>> On 7 December 2017 at 17:36, Kinney, Michael D
>> <michael.d.kinney@intel.com> wrote:
>> > Ard,
>> >
>> > With link time optimization, the current order produces
>> > smaller code.
>> >
>>
>> I don't think it does. You are essentially saying that
>> DebugAssertEnabled() may resolve to a link time constant
>> FALSE under
>> LTO.
>>
>> In that case, why would the following two statement not
>> be equivalent?
>>
>> if (FALSE && EFI_ERROR (StatusParameter)) {}
>>
>> if (EFI_ERROR (StatusParameter) && FALSE) {}
>>
>> (which is essentially what a nested if () resolves to)
>>
>> In other words, the compiler is smart enough to drop the
>> status check
>> in the second case, because it can see there are no side
>> effects, and
>> the condition can never be made true anyway.
>>
>> > Without link time optimization, your patch will produce
>> > smaller code, but not as small as link time optimized
>> code.
>> >