[llvm-dev] Early Tail Duplication Inefficiency

[Craig Topper via llvm-dev]

Try passing "-mllvm disable-early-taildup=true" to clang

~Craig


On Wed, Jan 30, 2019 at 10:01 AM Riyaz Puthiyapurayil via llvm-dev <
llvm-dev at lists.llvm.org> wrote:

> I didn’t see any response on this. Is there any way to turn off early tail
> duplication with a clang-7 option (other than completely turning off all
> optimizations)? The issue is reproducible with a very simple test case.
> Clang-7 with optimizations turned on takes hours compared to minutes with
> clang-5.0. Here is a simple cooked up test (the real-life example is of
> course different but this simple test exposes the same inefficiency):
>
>
>
> // test.cpp
>
> #include <string>
>
>
>
> struct Foo {
>
>     std::string s1;
>
>     std::string s2;
>
>     std::string s3;
>
> };
>
>
>
> Foo Array[] = {
>
>     { “0”, “0”, “0” },
>
>     { “1”, “1”, “1” },
>
>      :
>
>      :
>
>      :
>
>     { “9999”, “9999”, “9999” }
>
> };
>
>
>
> Compile:
>
>
>
> % clang++ -c -O3 test.cpp
>
> …
>
> Takes hours!
>
>
>
> *From:* Riyaz Puthiyapurayil
> *Sent:* Tuesday, January 29, 2019 11:27 AM
> *To:* 'llvm-dev' <llvm-dev at lists.llvm.org>
> *Subject:* Early Tail Duplication Inefficiency
>
>
>
> I have a file for which clang-7 takes over 2 hours to compile with -O3.
> For the same file, clang-5 takes less than 2 minutes (which is also high
> IMHO). I will try to create a test case (but it is pretty simple, it only
> contains initializations of many arrays of structs where the structs are of
> the following form:
>
>
>
> struct Foo {
>
>     EnumType1 e1; // there are 700+ enum labels
>
>     std::string s1;
>
>     EnumType2 e2; // 5 possible values for e2
>
>     std::string s2;
>
>     std::string s3;
>
> };
>
>
>
> // A large array with 10K+ elements
>
> Foo array1[] = {
>
>   { EnumType1Label1, “some string”, EnumType2Label1, “another string”,
> “yet another string” },
>
> :
>
> :
>
> };
>
>
>
> // 11 more arrays like above but most of them have only a few hundred
> elements
>
> :
>
> :
>
>
>
> I would like to know if a similar problem had been reported before. A
> quick search didn’t find anything…
>
>
>
> Clang-5 -ftime-report shows:
>
>
>
>
> ===-------------------------------------------------------------------------===
>
>                       ... Pass execution timing report ...
>
>
> ===-------------------------------------------------------------------------===
>
>   Total Execution Time: 175.9183 seconds (176.1324 wall clock)
>
>
>
>    ---User Time---   --System Time--   --User+System--   ---Wall Time---
> --- Name ---
>
>   38.9181 ( 22.4%)   0.0070 (  0.4%)  38.9251 ( 22.1%)  38.9712 ( 22.1%)
> Simple Register Coalescing
>
>   34.0788 ( 19.6%)   0.7859 ( 41.5%)  34.8647 ( 19.8%)  34.9069 ( 19.8%)
> SROA
>
>   18.8351 ( 10.8%)   0.0070 (  0.4%)  18.8421 ( 10.7%)  18.8652 ( 10.7%)
> Function Integration/Inlining
>
>   18.0393 ( 10.4%)   0.0010 (  0.1%)  18.0403 ( 10.3%)  18.0624 ( 10.3%)
> Branch Probability Basic Block Placement
>
>   17.5163 ( 10.1%)   0.3060 ( 16.2%)  17.8223 ( 10.1%)  17.8458 ( 10.1%)
> Merge disjoint stack slots
>
>   14.4318 (  8.3%)   0.0000 (  0.0%)  14.4318 (  8.2%)  14.4495 (  8.2%)
> Control Flow Optimizer
>
>    6.5960 (  3.8%)   0.6219 ( 32.9%)   7.2179 (  4.1%)   7.2315 (  4.1%)
> X86 DAG->DAG Instruction Selection
>
>    2.1577 (  1.2%)   0.0040 (  0.2%)   2.1617 (  1.2%)   2.1643 (  1.2%)
> Greedy Register Allocator
>
>    0.9539 (  0.5%)   0.0000 (  0.0%)   0.9539 (  0.5%)   0.9543 (  0.5%)
> Combine redundant instructions
>
>             :
>
>             :
>
>
>
> Clang-7 -ftime-report shows:
>
>
> ===-------------------------------------------------------------------------===
>
>                       ... Pass execution timing report ...
>
>
> ===-------------------------------------------------------------------------===
>
>   Total Execution Time: 7920.0840 seconds (8021.0405 wall clock)
>
>
>
>    ---User Time---   --System Time--   --User+System--   ---Wall Time---
> --- Name ---
>
>   6660.8174 ( 88.0%)  209.5201 ( 60.3%)  6870.3375 ( 86.7%)  6957.8224 (
> 86.7%)  Early Tail Duplication
>
>   674.2655 (  8.9%)   0.0550 (  0.0%)  674.3205 (  8.5%)  675.1329 (
> 8.4%)  Jump Threading
>
>   89.1534 (  1.2%)   8.1488 (  2.3%)  97.3022 (  1.2%)  97.6368 (  1.2%)
> Merge disjoint stack slots
>
>    2.4886 (  0.0%)  73.3249 ( 21.1%)  75.8135 (  1.0%)  79.9594 (  1.0%)
> Eliminate PHI nodes for register allocation
>
>    9.2116 (  0.1%)  52.7120 ( 15.2%)  61.9236 (  0.8%)  62.1655 (  0.8%)
> Slot index numbering
>
>   34.4118 (  0.5%)   2.5066 (  0.7%)  36.9184 (  0.5%)  44.2757 (  0.6%)
> SROA
>
>   35.2266 (  0.5%)   0.0000 (  0.0%)  35.2266 (  0.4%)  35.2803 (  0.4%)
> Simple Register Coalescing
>
>   18.0892 (  0.2%)   0.0020 (  0.0%)  18.0912 (  0.2%)  18.1253 (  0.2%)
> Function Integration/Inlining
>
>    7.2959 (  0.1%)   0.5739 (  0.2%)   7.8698 (  0.1%)   7.9301 (  0.1%)
> X86 DAG->DAG Instruction Selection
>
>    6.5990 (  0.1%)   0.0000 (  0.0%)   6.5990 (  0.1%)   6.6072 (  0.1%)
> Branch Probability Basic Block Placement
>
>    2.8736 (  0.0%)   0.0060 (  0.0%)   2.8796 (  0.0%)   2.8831 (  0.0%)
> Greedy Register Allocator
>
>    2.0147 (  0.0%)   0.1890 (  0.1%)   2.2037 (  0.0%)   2.2095 (  0.0%)
> Global Value Numbering
>
>    2.1347 (  0.0%)   0.0010 (  0.0%)   2.1357 (  0.0%)   2.1456 (  0.0%)
> Call-site splitting
>
>    1.5878 (  0.0%)   0.0010 (  0.0%)   1.5888 (  0.0%)   1.6079 (  0.0%)
> Combine redundant instructions
>
>    1.4358 (  0.0%)   0.0020 (  0.0%)   1.4378 (  0.0%)   1.4569 (  0.0%)
> Two-Address instruction pass
>
>    0.9689 (  0.0%)   0.0000 (  0.0%)   0.9689 (  0.0%)   0.9748 (  0.0%)
> Live Interval Analysis
>
>      :
>
>      :
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