https://llvm.org/bugs/show_bug.cgi?id=27110
Bug ID: 27110
Summary: Degraded speed of generated output
Product: clang
Version: trunk
Hardware: PC
OS: All
Status: NEW
Severity: normal
Priority: P
Component: LLVM Codegen
Assignee: unassignedclangb...@nondot.org
Reporter: 191...@gmail.com
CC: llvm-bugs@lists.llvm.org
Classification: Unclassified
Created attachment 16116
--> https://llvm.org/bugs/attachment.cgi?id=16116&action=edit
demo source code
I found there was a serious speed degrade for the output of clang when
compiling some specific kind of code since two weeks ago.
The attached file is a solver for 8-queen puzzle, it can be compiled with:
/opt/bin/clang -std=c99 -mssse3 -O3 -fomit-frame-pointer -fno-stack-protector
-fno-exceptions -o 8 8.c
Currently in my machine, it took 2.52s to run with eight queens:
$ time ./8 9
352 solutions
./8 9 2.52s user 0.01s system 99% cpu 2.545 total
But about two weeks ago, it took only ** 2.20s ** to finish the same input.
What is interesting is in the following function:
static inline int validate(const int* a, const int d)
{
for (int i = 0; i < d; ++i)
{
for (int j = i + 1, x = 1; j < d; ++j, ++x)
{
const int p = a[i] - a[j];
if (p == 0 || p == x || -p == x) return 0;
}
}
return 1;
}
Changing it to:
static inline int validate(const int* a, const int d)
{
for (int i = 0; i < d; ++i)
{
for (int j = i + 1, x = 1; j < d; ++j, ++x)
{
const int p = a[i] - a[j];
if (p == 0 || p == -x || p == -x) return 0; // <---
}
}
return 1;
}
could reduce the time from 2.52s to 2.31s (9% boost).
I checked the output assembly of gcc-5.3 and Intel compiler 2016.2, they both
did the optimization which splits the judgement statement to:
if (a[i] == a[j]) return 0;
const int p = a[i] - a[j];
if (p == -x || p == -x) return 0;
but clang didn't do this. If I manually rewrite the code, it took 2.15s (15%
boost).
---------
Comparing to gcc-5.3:
$ /opt/bin/gcc -std=c99 -mssse3 -O3 -fomit-frame-pointer -fno-stack-protector
-fno-exceptions -o 8 8.c
$ time ./8 9
352 solutions
./8 9 2.04s user 0.01s system 99% cpu 2.069 total
and Intel compiler 2016.2:
$ icc -std=c99 -mssse3 -O3 -fomit-frame-pointer -fno-stack-protector
-fno-exceptions -o 8 8.c
$ time ./8 9
352 solutions
./8 9 2.07s user 0.00s system 99% cpu 2.077 total
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