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experiment: try LTO for Linux native builds (ELF only) #72

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experiment: try LTO for Linux native builds (ELF only) #72

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18 changes: 16 additions & 2 deletions scripts/build-zstd.sh
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Original file line number Diff line number Diff line change
Expand Up @@ -110,11 +110,25 @@ case "$CLASSIFIER" in
*-aarch64) ARCH_FLAG="-mcpu=generic+crc" ;;
esac

# LTO: zig's Mach-O backend is a self-hosted linker with no LTO support
# ("error: LTO requires using LLD"), so this is ELF-only for now. zig 0.16
# still links ELF through its bundled real lld by default (the self-hosted
# ELF linker is early-stage/opt-in), so -flto works there — confirmed by
# cross-compiling this classifier to a .o and a linked .so locally. What
# can't be confirmed off a Linux host: that the LTO-linked library is
# actually correct at runtime, hence gating this on CI (which natively
# builds and test-runs linux-x86_64/linux-aarch64) rather than shipping it
# straight to every platform like the ASM/arch/hardening flags in #70.
LTO_FLAG=""
case "$CLASSIFIER" in
linux-*) LTO_FLAG="-flto" ;;
esac

# Decode (not encode) formats back to zstd v0.4, matching what zstd-jni
# ships by default. v0.1-v0.3 stay off (ZSTD_LEGACY_SUPPORT>=4 skips their
# zstd_v0{1,2,3}.c decoders per legacy/zstd_legacy.h) — those predate zstd's
# 1.0 stabilization and are vanishingly unlikely to show up in the wild.
CFLAGS="-O3 $ARCH_FLAG -DNDEBUG -DZSTD_LEGACY_SUPPORT=4 -DXXH_NAMESPACE=ZSTD_ $VIS_FLAG \
CFLAGS="-O3 $ARCH_FLAG $LTO_FLAG -DNDEBUG -DZSTD_LEGACY_SUPPORT=4 -DXXH_NAMESPACE=ZSTD_ $VIS_FLAG \
-I$ZSTD_LIB -I$ZSTD_LIB/common -fPIC"

i=0
Expand All @@ -131,7 +145,7 @@ wait
SONAME_FLAG=""
[ "$LIB_NAME" = "libzstd.so" ] && SONAME_FLAG="-Wl,-soname,libzstd.so.1"

zig cc -target "$ZIG_TARGET" -shared $STRIP_FLAG $SONAME_FLAG $LINK_EXTRA -o "$DEST_DIR/$LIB_NAME" "$WORK"/*.o
zig cc -target "$ZIG_TARGET" -shared $LTO_FLAG $STRIP_FLAG $SONAME_FLAG $LINK_EXTRA -o "$DEST_DIR/$LIB_NAME" "$WORK"/*.o

# lld emits a .pdb (debug database, multiple MB) and a .lib (import library) next
# to a Windows .dll; neither is needed at runtime and both would be bundled into
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