portage

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--- make.conf.example
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+++ make.conf.example
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@@ -22,6 +22,11 @@
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 # Example:
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 #USE="X gtk gnome -alsa"
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+# Host Setting
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+# ============
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+#
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+CHOST="x86_64-pc-linux-gnu"
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+
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 # Host and optimization settings
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 # ==============================
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 #
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@@ -39,10 +44,35 @@
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 # -frecord-gcc-switches, since otherwise the check could result in false
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 # positive results.
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 #
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-# Please refer to the GCC manual for a list of possible values.
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+# -mcpu=<cpu-type> means optimize code for the particular type of CPU without
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+# breaking compatibility with other CPUs. GCC 3.4 has deprecated support for
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+# -mcpu, so use -mtune instead if using this compiler.
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+#
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+# -march=<cpu-type> means to take full advantage of the ABI and instructions
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+# for the particular CPU; this will break compatibility with older CPUs (for
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+# example, -march=athlon-xp code will not run on a regular Athlon, and
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+# -march=i686 code will not run on a Pentium Classic.)
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+#
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+# CPU types supported in gcc-3.2 and higher: athlon-xp, athlon-mp,
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+# athlon-tbird, athlon, k6, k6-2, k6-3, i386, i486, i586 (Pentium), i686
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+# (PentiumPro), pentium, pentium-mmx, pentiumpro, pentium2 (Celeron),
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+# pentium3, and pentium4.
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+#
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+# Note that Gentoo Linux 1.4 and higher include at least gcc-3.2.
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+#
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+# amd64 CPU types supported in gcc-3.4: athlon64, opteron, k8
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+# 
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+# CRITICAL WARNINGS: ****************************************************** #
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+# K6 markings are deceptive. Avoid setting -march for them. See Bug #24379. #
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+# Pentium-M CPUs should not enable sse2 until at least gcc-3.4. Bug 50616. #
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+# GCC 3.3 doesnt support an amd64 specific -march setting, use 3.4.         #
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+# ************************************************************************* #
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 #
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-#CFLAGS="-O2 -pipe"
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+# Decent examples:
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 #
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+#CFLAGS="-mtune=k8 -O2 -pipe"
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+#CFLAGS="-march=athlon64 -O2 -pipe"
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+
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 # If you set a CFLAGS above, then this line will set your default C++ flags to
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 # the same settings.
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 #CXXFLAGS="${CFLAGS}"
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@@ -76,7 +106,7 @@
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 # DO NOT PUT ANYTHING BUT YOUR SPECIFIC ~ARCHITECTURE IN THE LIST.
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 # IF YOU ARE UNSURE OF YOUR ARCH, OR THE IMPLICATIONS, DO NOT MODIFY THIS.
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 #
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-#ACCEPT_KEYWORDS="~arch"
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+#ACCEPT_KEYWORDS="~amd64"
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 # ACCEPT_LICENSE is used to mask packages based on licensing restrictions.
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 # It may contain both license and group names, where group names are
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