[llvm-dev] Aggressive optimization opportunity (original) (raw)

Zheng CZ Chen via llvm-dev llvm-dev at lists.llvm.org
Tue Jan 15 18:56:45 PST 2019

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

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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