[cfe-dev] Fw: [llvm-dev] Aggressive optimization opportunity
Zheng CZ Chen via cfe-dev
cfe-dev at lists.llvm.org
Thu Jan 17 04:20:20 PST 2019
Hi guys,
I posted this discussion and some one said this should be discussed in
cfe-dev. So forward it here, sorry for the long mail.
My question is: is it possible to add a clang option to make cfe set all
function pointer parameters with restrict attribute as if user add a
keyword 'restrict' for this parameter?
Reason why I make this proposal is because some compiler like IBM XL has
similar option -qrestrict for performance tuning.
If we confirm functionality is right, adding this kind of option will make
application run faster. There is a small example in my first mail.
Thanks in advance and look forward to your answer.
BRS//
Chen Zheng
Power Compiler Backend Developer
----- Forwarded by Zheng CZ Chen/China/IBM on 2019/01/17 08:11 PM -----
From: Chandler Carruth <chandlerc at gmail.com>
To: Zheng CZ Chen <czhengsz at cn.ibm.com>
Cc: Troy Johnson <troyj at cray.com>, "Finkel, Hal J."
<hfinkel at anl.gov>, Roman Lebedev <lebedev.ri at gmail.com>,
"llvm-dev at lists.llvm.org" <llvm-dev at lists.llvm.org>
Date: 2019/01/16 12:20 PM
Subject: Re: [llvm-dev] Aggressive optimization opportunity
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
Inactive hide details for Troy Johnson ---2019/01/16 02:32:22
AM---Restrict is supported by Clang for C++ via __restrict__, soTroy
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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Lead, Compiler Technology and Programming Languages
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