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Date: Sun, 13 Mar 2022 03:17:35 +0900
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To: PHP internals <internals@lists.php.net>
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Subject: LTO support
From: zeriyoshi@gmail.com (Go Kudo)

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At present, PHP cannot be built using LTO (Link Time Optimization).

LTO is a compiler feature that can improve performance by optimizing at
link time.
Chromium is also using this feature.

https://gcc.gnu.org/onlinedocs/gccint/LTO-Overview.html

The reason for the LTO failure appears to be the Zend Engine's use of GCC's
global register variables. Conversely, it is otherwise compatible with LTO.
(except: opcache).

I modified toolchain too to enable and validate LTOs outside of Zend Engine
and OPcache:

https://github.com/zeriyoshi/php-src/commit/b25c237837fec2f82d268d8dbd45ec886baf474f

I tested the following compilation options:

```
gcc: (Debian 10.2.1-6) 10.2.1 20210110
CFLAGS="-fstack-protector-strong -fpic -fpie -O3 -D_LARGEFILE_SOURCE
-D_FILE_OFFSET_BITS=64"
CXXFLAGS="${CFLAGS}"
LDFLAGS="-Wl,-O1 -pie"
$ ./sapi/cli/php ./Zend/bench.php
simple             0.010
simplecall         0.005
simpleucall        0.010
simpleudcall       0.013
mandel             0.052
mandel2            0.049
ackermann(7)       0.012
ary(50000)         0.003
ary2(50000)        0.002
ary3(2000)         0.020
fibo(30)           0.035
hash1(50000)       0.004
hash2(500)         0.004
heapsort(20000)    0.012
matrix(20)         0.012
nestedloop(12)     0.010
sieve(30)          0.006
strcat(200000)     0.002
------------------------
Total              0.261
```

vs

```
gcc: (Debian 10.2.1-6) 10.2.1 20210110
CFLAGS="-fstack-protector-strong -fpic -fpie -O3 -flto -fuse-linker-plugin
-D_LARGEFILE_SOURCE -D_FILE_OFFSET_BITS=64"
CXXFLAGS="${CFLAGS}"
LDFLAGS="-Wl,-O1 -pie -flto -fuse-linker-plugin"
$ ./sapi/cli/php ./Zend/bench.php
simple             0.006
simplecall         0.003
simpleucall        0.010
simpleudcall       0.014
mandel             0.050
mandel2            0.049
ackermann(7)       0.012
ary(50000)         0.003
ary2(50000)        0.002
ary3(2000)         0.020
fibo(30)           0.035
hash1(50000)       0.004
hash2(500)         0.004
heapsort(20000)    0.012
matrix(20)         0.011
nestedloop(12)     0.010
sieve(30)          0.007
strcat(200000)     0.002
------------------------
Total              0.255
```

In fact, it does not seem to work very well. However, it may be effective
when large numbers of extensions are built into PHP.

Is anyone interested in supporting this?

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