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Date: Fri, 23 Oct 2015 14:30:26 +0200
From: Denys Vlasenko <vda.linux@...glemail.com>
To: Rich Felker <dalias@...c.org>
Cc: Denys Vlasenko <vda.linux@...glemail.com>, musl <musl@...ts.openwall.com>
Subject: [PATCH] configure: add gcc flags for better link-time optimization
libc.so size reduction:
text data bss dec hex filename
564099 1944 11768 577811 8d113 libc.so.before
562277 1924 11576 575777 8c921 libc.so
Signed-off-by: Denys Vlasenko <vda.linux@...glemail.com>
CC: musl <musl@...ts.openwall.com>
---
configure | 38 ++++++++++++++++++++++++++++++++++++++
1 file changed, 38 insertions(+)
diff --git a/configure b/configure
index 4564ad8..a9ed159 100755
--- a/configure
+++ b/configure
@@ -440,6 +440,44 @@ tryflag CFLAGS_AUTO -fno-unwind-tables
tryflag CFLAGS_AUTO -fno-asynchronous-unwind-tables
#
+# When linker merges sections, a tiny section (such as one resulting
+# from "static char flag_var") with no alignment restrictions
+# can end up logded between two more strongly aligned ones (say,
+# "static int global_cnt1/2", both of which want 32-bit alignment).
+# Then this byte-sized "flag_var" gets 3 bytes of padding.
+#
+# With section sorting by alignment, one-byte flag variables have
+# higher chance of being grouped together and not require padding.
+# (It can be made even better. Linker is too dumb.
+# ld needs to grow -Wl,--pack-sections-optimally)
+#
+# For us, this affects the size of only one file: libc.so
+#
+tryldflag LDFLAGS_AUTO -Wl,--sort-section=alignment
+tryldflag LDFLAGS_AUTO -Wl,--sort-common
+
+#
+# Put every function and data object into its own section:
+# .text.funcname, .data.var, .rodata.const_struct, .bss.zerovar
+#
+# Previous optimization isn't working too well by itself
+# because data objects aren't living in separate sections,
+# they are all grouped in one .data and one .bss section per *.o file.
+# With -ffunction/data-sections, section sorting eliminates more padding.
+#
+# Object files in static *.a files will also have their functions
+# and data objects each in its own section.
+#
+# This enables programs statically linked with -Wl,--gc-sections
+# to perform "section garbage collection": drop unused code and data
+# not on per-*.o-file basis, but on per-function and per-object basis.
+# This is a big thing: --gc-sections sometimes eliminates several percent
+# of unreachable code and data in final executable.
+#
+tryflag CFLAGS_AUTO -ffunction-sections
+tryflag CFLAGS_AUTO -fdata-sections
+
+#
# The GNU toolchain defaults to assuming unmarked files need an
# executable stack, potentially exposing vulnerabilities in programs
# linked with such object files. Fix this.
--
1.8.1.4
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