[llvm-dev] Aggressive optimization opportunity
Chandler Carruth via llvm-dev
llvm-dev at lists.llvm.org
Tue Jan 15 20:20:08 PST 2019
This isn't really a discussion about LLVM any longer. LLVM has the ability
to model pointer parameters with the desired semantics.
This is now a question about a C/C++ extension to Clang. That discussion
would probably be best held on cfe-dev where there are more folks focused
on the language and frontend. My suspicion is that there will be very
little desire for an extension that knowingly and intentionally breaks very
basic properties of C and C++, but it may be worth asking the correct
audience that question. =]
On Tue, Jan 15, 2019 at 6:57 PM Zheng CZ Chen via llvm-dev <
llvm-dev at lists.llvm.org> wrote:
> Hi,
>
> Very appreciate for all your input.
>
> it is indeed a very aggressive optimization and it is not safe in many
> cases. But it should be helpful to tune application's performance if it is
> safe.
>
> I think if we want to support it, we must:
> 1: only let compiler user turn it on by explicitly specifying
> -fforce-restrict-ptr-args, otherwise it is always off.
> 2: emit a warning message to remind this opt will change program semantics
> if users turn it on.
> 3: restrict its application to C/C++.
>
> Any ideas?
>
> Thanks.
>
> BRS//
> Chen Zheng
> Power Compiler Backend Developer
>
>
> [image: Inactive hide details for Troy Johnson ---2019/01/16 02:32:22
> AM---Restrict is supported by Clang for C++ via __restrict__, so]Troy
> Johnson ---2019/01/16 02:32:22 AM---Restrict is supported by Clang for C++
> via __restrict__, so it seems strange to block using this pro
>
> From: Troy Johnson <troyj at cray.com>
> To: "Finkel, Hal J." <hfinkel at anl.gov>, Zheng CZ Chen <czhengsz at cn.ibm.com
> >
> Cc: "llvm-dev at lists.llvm.org" <llvm-dev at lists.llvm.org>
> Date: 2019/01/16 02:32 AM
> Subject: RE: [llvm-dev] Aggressive optimization opportunity
> ------------------------------
>
>
>
> Restrict is supported by Clang for C++ via __restrict__, so it seems
> strange to block using this proposed option for C++.
>
> That said, this kind of option can be dangerous and should come with a
> suitable warning. We’ve had a similar option and in practice it’s been used
> to hunt for performance gains (i.e., turn it on and see what happens), but
> just because the code runs faster and produces the correct result with the
> option enabled doesn’t mean it is safe in all cases. And if it crashes or
> gives you wrong answers, you still don’t know which pointer had the alias
> that caused that problem. Either way, you still need to inspect all of the
> pointers and prove to yourself it is safe and at that point you might as
> well add restrict manually.
>
> -Troy
>
> *From:* llvm-dev <llvm-dev-bounces at lists.llvm.org> *On Behalf Of *Finkel,
> Hal J. via llvm-dev
> *Sent:* Tuesday, January 15, 2019 9:57 AM
> *To:* Zheng CZ Chen <czhengsz at cn.ibm.com>; llvm-dev at lists.llvm.org
> *Subject:* Re: [llvm-dev] Aggressive optimization opportunity
>
>
> On 1/15/19 6:07 AM, Zheng CZ Chen via llvm-dev wrote:
>
> Hi,
>
> There are some compilers with a aggressive optimization which
> restricts function pointer parameters. Let's say opt restrict_args. When
> restrict_args is turned on, compiler will treat all function pointer
> parameters as restrict one.
>
> I certainly understand the use case, in a general sense. In my experience,
> these options are used with (fairly old) pre-C99 code bases (and
> specifically C, not C++), which follow something akin to a
> one-function-per-source-file model and which can't be modified (e.g., for
> licensing reasons). Using these options are certainly considered bad
> practice, and they only apply to certain legacy code bases. Does this match
> your experience and expected usage?
>
> In an engineering sense, this seems like a trivial feature to support. I
> don't object to supporting it, but if we do, we probably want to:
>
> 1. Restrict it's application to C (e.g., it should be an error to use with
> C++, OpenCL, CUDA, and any other languages that Clang supports).
>
> 2. When used with C99 or later language standards, the use of this flag
> generates a warning on each function definition with a fixit hint showing
> where the restrict keyword should be placed (we can then, optionally of
> course, use these fixits to automatically upgrade code where possible using
> our corresponding infrastructure). This warning should have a separate
> flag, and is disabled by default for pre-C99 standard modes, and enabled by
> default otherwise, but can be toggled independently.
>
> -Hal
>
>
> * int foo(int * a) *+ restrict_args opt
>
> equals to:
>
> * int foo(int * restrict a)*
>
>
> Here is a complete example:
> source code:
> extern int num;
>
>
>
>
> * int foo(int * a) { (*a) = 10; num++; (*a)++;*
>
>
> * return *a; }*
>
> Using IBM xlc compiler with option -qrestrict at -O2, we get result:
>
> 0000000000000000 <foo>:
> 0: 00 00 4c 3c addis r2,r12,0
> 4: 00 00 42 38 addi r2,r2,0
> 8: 00 00 a2 3c addis r5,r2,0
> c: 00 00 a5 e8 ld r5,0(r5)
> 10: 0b 00 00 38 li r0,11
> 14: 00 00 03 90 stw r0,0(r3)
> 18: 00 00 85 80 lwz r4,0(r5)
> * 1c: 0b 00 60 38 li r3,11 ------>since we confirm num will not change the
> content where pointer to, compiler can directly return 11.*
> 20: 01 00 04 38 addi r0,r4,1
> 24: 00 00 05 90 stw r0,0(r5)
> 28: 20 00 80 4e blr
>
> Seems clang does not have such optimization. And I don't find
> similar option in gcc either.
>
> Is it possible to add this optimization into clang?
>
> Thanks.
>
> BRS//
> Chen Zheng
> Power Compiler Backend Developer
>
>
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> --
> Hal Finkel
> Lead, Compiler Technology and Programming Languages
> Leadership Computing Facility
> Argonne National Laboratory
>
>
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