https://gcc.gnu.org/bugzilla/show_bug.cgi?id=61958
Bug ID: 61958
Summary: function arbitrarily placed in .text.unlikely section
Product: gcc
Version: 4.9.1
Status: UNCONFIRMED
Severity: normal
Priority: P3
Component: c
Assignee: unassigned at gcc dot gnu.org
Reporter: jpoimboe at redhat dot com
Created attachment 33206
--> https://gcc.gnu.org/bugzilla/attachment.cgi?id=33206&action=edit
gzipped .i file for the bad case
When making changes to a certain source file in the Linux kernel, gcc is
unexpectedly moving two functions from the .text section to the .text.unlikely
section.
When compiling the original version of the source file, the two functions are
placed in the .text section:
$ cd linux
$ git describe
v3.16-rc7-7-g31dab71
$ make net/ipv4/fib_trie.o
CHK include/config/kernel.release
CHK include/generated/uapi/linux/version.h
CHK include/generated/utsrelease.h
CALL scripts/checksyscalls.sh
CC net/ipv4/fib_trie.o
$ eu-readelf -s net/ipv4/fib_trie.o |grep 'insert_leaf_info\|leaf_info_new'
6: 0000000000000000 136 FUNC LOCAL DEFAULT 1 insert_leaf_info
37: 0000000000000dc0 88 FUNC LOCAL DEFAULT 1 leaf_info_new
$ eu-readelf -S net/ipv4/fib_trie.o |grep '\[ 1\]'
[ 1] .text PROGBITS 0000000000000000 00000040 000038c0 0 AX
0 0 16
However, when I apply the following patch, the two functions get unexpectedly
moved to .text.unlikely:
$ cat /tmp/fib_trie.patch
diff --git a/net/ipv4/fib_trie.c b/net/ipv4/fib_trie.c
index 5afeb5a..cc5e3d3 100644
--- a/net/ipv4/fib_trie.c
+++ b/net/ipv4/fib_trie.c
@@ -1187,6 +1187,7 @@ int fib_table_insert(struct fib_table *tb, struct
fib_config *cfg)
key = key & mask;
+ printk("foo\n");
fi = fib_create_info(cfg);
if (IS_ERR(fi)) {
err = PTR_ERR(fi);
$ patch -p1 < /tmp/fib_trie.patch
patching file net/ipv4/fib_trie.c
$ make net/ipv4/fib_trie.o
CHK include/config/kernel.release
CHK include/generated/uapi/linux/version.h
CHK include/generated/utsrelease.h
CALL scripts/checksyscalls.sh
CC net/ipv4/fib_trie.o
$ eu-readelf -s net/ipv4/fib_trie.o |grep 'insert_leaf_info\|leaf_info_new'
6: 0000000000000000 111 FUNC LOCAL DEFAULT 5 insert_leaf_info
28: 000000000000006f 82 FUNC LOCAL DEFAULT 5 leaf_info_new
$ eu-readelf -S net/ipv4/fib_trie.o |grep '\[ 5\]'
[ 5] .text.unlikely PROGBITS 0000000000000000 00003770 000000c1 0 AX
0 0 1
Both of the moved functions are called by fib_insert_node(), which is inlined
by fib_table_insert(), which was modified by the patch. As far as I can tell,
it seems likely that the moved functions would be called, especially
insert_leaf_info() which is called in the main control path of the function.
Also it seems odd that adding a call to printk would change the likelihood of a
function being called, since it doesn't change control flow.
Using gcc from Fedora rawhide:
$ gcc -v
Using built-in specs.
COLLECT_GCC=gcc
COLLECT_LTO_WRAPPER=/usr/libexec/gcc/x86_64-redhat-linux/4.9.1/lto-wrapper
Target: x86_64-redhat-linux
Configured with: ../configure --prefix=/usr --mandir=/usr/share/man
--infodir=/usr/share/info --with-bugurl=http://bugzilla.redhat.com/bugzilla
--enable-bootstrap --enable-shared --enable-threads=posix
--enable-checking=release --enable-multilib --with-system-zlib
--enable-__cxa_atexit --disable-libunwind-exceptions --enable-gnu-unique-object
--enable-linker-build-id --with-linker-hash-style=gnu
--enable-languages=c,c++,objc,obj-c++,fortran,ada,go,lto --enable-plugin
--enable-initfini-array --disable-libgcj
--with-isl=/builddir/build/BUILD/gcc-4.9.1-20140717/obj-x86_64-redhat-linux/isl-install
--with-cloog=/builddir/build/BUILD/gcc-4.9.1-20140717/obj-x86_64-redhat-linux/cloog-install
--enable-gnu-indirect-function --with-tune=generic --with-arch_32=i686
--build=x86_64-redhat-linux
Thread model: posix
gcc version 4.9.1 20140717 (Red Hat 4.9.1-2) (GCC)
Full gcc cmdline:
gcc -Wp,-MD,net/ipv4/.fib_trie.o.d -nostdinc -isystem
/usr/lib/gcc/x86_64-redhat-linux/4.9.1/include -I./arch/x86/include
-Iarch/x86/include/generated -Iinclude -I./arch/x86/include/uapi
-Iarch/x86/include/generated/uapi -I./include/uapi -Iinclude/generated/uapi
-include ./include/linux/kconfig.h -D__KERNEL__ -Wall -Wundef
-Wstrict-prototypes -Wno-trigraphs -fno-strict-aliasing -fno-common
-Werror-implicit-function-declaration -Wno-format-security -m64 -mno-mmx
-mno-sse -mno-80387 -mno-fp-ret-in-387 -mpreferred-stack-boundary=3
-mtune=generic -mno-red-zone -mcmodel=kernel -funit-at-a-time
-maccumulate-outgoing-args -DCONFIG_AS_CFI=1 -DCONFIG_AS_CFI_SIGNAL_FRAME=1
-DCONFIG_AS_CFI_SECTIONS=1 -DCONFIG_AS_FXSAVEQ=1 -DCONFIG_AS_CRC32=1
-DCONFIG_AS_AVX=1 -DCONFIG_AS_AVX2=1 -pipe -Wno-sign-compare
-fno-asynchronous-unwind-tables -mno-sse -mno-mmx -mno-sse2 -mno-3dnow -mno-avx
-fno-delete-null-pointer-checks -O2 -Wframe-larger-than=2048
-fstack-protector-strong -Wno-unused-but-set-variable -fno-omit-frame-pointer
-fno-optimize-sibling-calls -fno-var-tracking-assignments -g -pg -mfentry
-DCC_USING_FENTRY -Wdeclaration-after-statement -Wno-pointer-sign
-fno-strict-overflow -fconserve-stack -Werror=implicit-int
-Werror=strict-prototypes -Werror=date-time -DCC_HAVE_ASM_GOTO
-D"KBUILD_STR(s)=#s" -D"KBUILD_BASENAME=KBUILD_STR(fib_trie)"
-D"KBUILD_MODNAME=KBUILD_STR(fib_trie)" -c -o net/ipv4/fib_trie.o
net/ipv4/fib_trie.c
I have also seen this issue with "gcc version 4.8.3 20140624 (Red Hat 4.8.3-1)
(GCC)" from Fedora 20.
The .i files are attached (and gzipped to be under the bugzilla file size
limit).
