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Re: Compiling c++ template is very slow.
On Fri, Mar 9, 2018 at 11:32 AM, Richard Biener
<richard.guenther@gmail.com> wrote:
> On Fri, Mar 9, 2018 at 3:33 AM, Fis Trivial <ybbs.daans@hotmail.com> wrote:
>>
>> I tried to use c++ template to generate code for a personal project, but
>> found that the compilation time needed with g++ is much
>> slower(exponentially) than with clang++.
>>
>> This is a code snippet for testing purpose:
>>
>> #include <iostream>
>>
>> template <int a, int b>
>> struct v : v<a-1, b>, v<a, b-1>
>> {
>> static int constexpr m = a;
>> static int constexpr n = b;
>> static int constexpr s = a + b;
>> };
>>
>> template <int b>
>> struct v<1, b> : v<1, b-1>
>> {
>> static int constexpr m = 1;
>> static int constexpr n = b;
>> static int constexpr s = b + 1;
>> };
>> template <int a>
>> struct v<a, 1> : v<a-1, 1>
>> {
>> static int constexpr m = a;
>> static int constexpr n = 1;
>> static int constexpr s = a + 1;
>> };
>> template <>
>> struct v<1, 1>
>> {
>> static int constexpr m = 1;
>> static int constexpr n = 1;
>> static int constexpr s = 2;
>> };
>>
>> int main()
>> {
>> std::cout << v<7, 12>::s << std::endl;
>> std::cout << v<4, 3>::s << std::endl;
>> };
>>
>>
>> Here is the time information:
>> ---
>> $ time g++ -std=c++11 generate.cc -o bygcc
>>
>> real 0m39.529s
>> user 0m39.418s
>> sys 0m0.053s
>>
>> $ time clang++ -std=c++11 generate.cc -o byclang
>>
>> real 0m0.310s
>> user 0m0.273s
>> sys 0m0.024s
>> ---
>>
>> When using greater value, gcc will require exponentially more time to
>> compile while the needed time from clang grows linearly. For example,
>> replacing the `main` function from above code to:
>>
>> int main()
>> {
>> // change 7 to 8 and drop <4, 3>
>> std::cout << v<8, 12>::s << std::endl;
>> };
>>
>> ---
>> $ time g++ -std=c++11 generate.cc -o bygcc
>>
>> real 5m20.755s
>> user 5m8.509s
>> sys 0m0.260s
>
> I can confirm this and suggest to open a bugreport.
callgrind shows that propagate_binfo_offsets recursing
to self very many times is likely the issue. Your templates
build a very deep inheritance chain and it seems that
the binfo offset propagation ends up being exponential here.
I would have expected that any bases have already correct
offsets so we don't need the recursion to bases of bases?
Ah, so in this case we have offset of 1 because sizeof is
always nonzero. So the issue might just be that
BINFO_OFFSET is not relative to the parent?
Anyway,
Index: gcc/cp/class.c
===================================================================
--- gcc/cp/class.c (revision 258380)
+++ gcc/cp/class.c (working copy)
@@ -5755,6 +5755,9 @@ propagate_binfo_offsets (tree binfo, tre
tree primary_binfo;
tree base_binfo;
+ if (integer_zerop (offset))
+ return;
+
/* Update BINFO's offset. */
BINFO_OFFSET (binfo)
= fold_convert (sizetype,
doens't improve things. We call this functions millions of times
creating millions of INTEGER_CSTs. Can BINFO_OFFSET be
non-constant?
Richard.
>> $ time clang++ -std=c++11 generate.cc -o byclang
>>
>> real 0m0.314s
>> user 0m0.281s
>> sys 0m0.020s
>> ---
>>
>>
>> Just for fun, making the template parameter to 128:
>> ---
>> $ time g++ -std=c++11 generate.cc -o bygcc
>> ... not gonna happen :).
>>
>> $ time clang++ -std=c++11 generate.cc -o byclang
>>
>> real 0m18.549s
>> user 0m18.410s
>> sys 0m0.066s
>> ---
>>
>>
>> I am currently running Fedora 27 with following version of gcc and
>> clang:
>>
>> $ g++ --version
>> g++ (GCC) 7.3.1 20180303 (Red Hat 7.3.1-5)
>> Copyright (C) 2017 Free Software Foundation, Inc.
>> This is free software; see the source for copying conditions. There is NO
>> warranty; not even for MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.
>>
>> $ clang++ --version
>> clang version 5.0.1 (tags/RELEASE_501/final)
>> Target: x86_64-unknown-linux-gnu
>> Thread model: posix
>> InstalledDir: /usr/bin