diff mbox series

[v3] Use __executable_start as the lowest address for profiling [BZ #28153]

Message ID 20210805120904.3528530-1-hjl.tools@gmail.com
State Committed
Commit 84a7eb1f87c1d01b58ad887a0ab5d87abbc1c772
Headers show
Series [v3] Use __executable_start as the lowest address for profiling [BZ #28153] | expand

Checks

Context Check Description
dj/TryBot-apply_patch success Patch applied to master at the time it was sent
dj/TryBot-32bit success Build for i686

Commit Message

H.J. Lu Aug. 5, 2021, 12:09 p.m. UTC
Glibc assumes that ENTRY_POINT is the lowest address for which we need
to keep profiling records and BFD linker uses a linker script to place
the input sections.

Starting from GCC 4.6, the main function is placed in .text.startup
section and starting from binutils 2.22, BFD linker with

commit add44f8d5c5c05e08b11e033127a744d61c26aee
Author: Alan Modra <amodra@gmail.com>
Date:   Thu Nov 25 03:03:02 2010 +0000

            * scripttempl/elf.sc: Group .text.exit, text.startup and .text.hot
            sections.

places .text.startup section before .text section, which leave the main
function out of profiling records.

Starting from binutils 2.15, linker provides __executable_start to mark
the lowest address of the executable.  Use __executable_start as the
lowest address to keep the main function in profiling records. This fixes
[BZ #28153].

Tested on Linux/x86-64, Linux/x32 and Linux/i686 as well as with
build-many-glibcs.py.
---
 csu/gmon-start.c              | 10 +++++++++-
 gmon/tst-gmon-gprof.sh        |  2 ++
 gmon/tst-gmon-static-gprof.sh |  2 ++
 3 files changed, 13 insertions(+), 1 deletion(-)

Comments

Fāng-ruì Sòng Aug. 6, 2021, 12:16 a.m. UTC | #1
On 2021-08-05, H.J. Lu via Libc-alpha wrote:
>Glibc assumes that ENTRY_POINT is the lowest address for which we need
>to keep profiling records and BFD linker uses a linker script to place
>the input sections.
>
>Starting from GCC 4.6, the main function is placed in .text.startup
>section and starting from binutils 2.22, BFD linker with
>
>commit add44f8d5c5c05e08b11e033127a744d61c26aee
>Author: Alan Modra <amodra@gmail.com>
>Date:   Thu Nov 25 03:03:02 2010 +0000
>
>            * scripttempl/elf.sc: Group .text.exit, text.startup and .text.hot
>            sections.
>
>places .text.startup section before .text section, which leave the main
>function out of profiling records.
>
>Starting from binutils 2.15, linker provides __executable_start to mark
>the lowest address of the executable.  Use __executable_start as the
>lowest address to keep the main function in profiling records. This fixes
>[BZ #28153].
>
>Tested on Linux/x86-64, Linux/x32 and Linux/i686 as well as with
>build-many-glibcs.py.
>---
> csu/gmon-start.c              | 10 +++++++++-
> gmon/tst-gmon-gprof.sh        |  2 ++
> gmon/tst-gmon-static-gprof.sh |  2 ++
> 3 files changed, 13 insertions(+), 1 deletion(-)

LGTM
Fāng-ruì Sòng Aug. 6, 2021, 7:21 a.m. UTC | #2
On 2021-08-05, Fangrui Song wrote:
>
>On 2021-08-05, H.J. Lu via Libc-alpha wrote:
>>Glibc assumes that ENTRY_POINT is the lowest address for which we need
>>to keep profiling records and BFD linker uses a linker script to place
>>the input sections.
>>
>>Starting from GCC 4.6, the main function is placed in .text.startup
>>section and starting from binutils 2.22, BFD linker with
>>
>>commit add44f8d5c5c05e08b11e033127a744d61c26aee
>>Author: Alan Modra <amodra@gmail.com>
>>Date:   Thu Nov 25 03:03:02 2010 +0000
>>
>>           * scripttempl/elf.sc: Group .text.exit, text.startup and .text.hot
>>           sections.
>>
>>places .text.startup section before .text section, which leave the main
>>function out of profiling records.
>>
>>Starting from binutils 2.15, linker provides __executable_start to mark
>>the lowest address of the executable.  Use __executable_start as the
>>lowest address to keep the main function in profiling records. This fixes
>>[BZ #28153].
>>
>>Tested on Linux/x86-64, Linux/x32 and Linux/i686 as well as with
>>build-many-glibcs.py.
>>---
>>csu/gmon-start.c              | 10 +++++++++-
>>gmon/tst-gmon-gprof.sh        |  2 ++
>>gmon/tst-gmon-static-gprof.sh |  2 ++
>>3 files changed, 13 insertions(+), 1 deletion(-)
>
>LGTM

[I had not subscribed the list until few days ago]

Comment to v2: gold/ld.lld don't necessarily place .text.unlikely
before other text sections.
GNU ld's fixed section ordering may make certain optimization difficult.

RISC architectures typically create range extension thunks to overcome the
limitation of short range branches. Hot code can usually be moved to the
middle to increase the chance that one range extension thunk can be
shared by more code.

8MiB cold
2MiB hot
8MiB cold

(
.text.sorted.* is a recent addition emulation link-time layout for
improving instruction cache and TLB. An optimized case may require the
sections to be interleaved with others.)
diff mbox series

Patch

diff --git a/csu/gmon-start.c b/csu/gmon-start.c
index b3432885b3..344606a676 100644
--- a/csu/gmon-start.c
+++ b/csu/gmon-start.c
@@ -52,6 +52,11 @@  extern char ENTRY_POINT[];
 #endif
 extern char etext[];
 
+/* Use __executable_start as the lowest address to keep profiling records
+   if it provided by the linker.  */
+extern const char executable_start[] asm ("__executable_start")
+  __attribute__ ((weak, visibility ("hidden")));
+
 #ifndef TEXT_START
 # ifdef ENTRY_POINT_DECL
 #  define TEXT_START ENTRY_POINT
@@ -92,7 +97,10 @@  __gmon_start__ (void)
   called = 1;
 
   /* Start keeping profiling records.  */
-  __monstartup ((u_long) TEXT_START, (u_long) &etext);
+  if (&executable_start != NULL)
+    __monstartup ((u_long) &executable_start, (u_long) &etext);
+  else
+    __monstartup ((u_long) TEXT_START, (u_long) &etext);
 
   /* Call _mcleanup before exiting; it will write out gmon.out from the
      collected data.  */
diff --git a/gmon/tst-gmon-gprof.sh b/gmon/tst-gmon-gprof.sh
index 9d371582b9..dc0be02110 100644
--- a/gmon/tst-gmon-gprof.sh
+++ b/gmon/tst-gmon-gprof.sh
@@ -39,12 +39,14 @@  trap cleanup 0
 cat > "$expected" <<EOF
 f1 2000
 f2 1000
+f3 1
 EOF
 
 # Special version for powerpc with function descriptors.
 cat > "$expected_dot" <<EOF
 .f1 2000
 .f2 1000
+.f3 1
 EOF
 
 "$GPROF" -C "$program" "$data" \
diff --git a/gmon/tst-gmon-static-gprof.sh b/gmon/tst-gmon-static-gprof.sh
index 79218df967..4cc99c80d0 100644
--- a/gmon/tst-gmon-static-gprof.sh
+++ b/gmon/tst-gmon-static-gprof.sh
@@ -39,6 +39,7 @@  trap cleanup 0
 cat > "$expected" <<EOF
 f1 2000
 f2 1000
+f3 1
 main 1
 EOF
 
@@ -46,6 +47,7 @@  EOF
 cat > "$expected_dot" <<EOF
 .f1 2000
 .f2 1000
+.f3 1
 .main 1
 EOF