Hi,
I would like to ask some feedback on the attached patch, which cleans up
the kernel optimization options for amd64. This was touched upon
earlier by Alexander Best in freebsd-toolchain, here:
http://lists.freebsd.org/pipermail/freebsd-toolchain/2011-October/000270.html
What this patch attempts to fix is the following:
- When you compile amd64 kernels for debug, they still get optimized
with "-O2 -frename-registers", which is almost certain to make
debugging miserable. Optimizing at higher levels makes variables and
code move around, or disappear altogether. About -frename-registers
the gcc documentation even says: "Depending on the debug information
format adopted by the target, however, it can make debugging
impossible, since variables will no longer stay in a “home register”."
- Clang doesn't support the -frename-registers option, so you get
harmless but annoying "warning: argument unused during compilation:
'-frename-registers'" messages during buildkernel.
The patch makes it so that:
- For normal amd64 kernel builds, it uses "-O2 -frename-registers",
unless Clang is used, then it uses plain "-O2".
- For debug amd64 kernel builds, it uses "-O", just like all the other
arches.
Index: sys/conf/kern.pre.mk
===================================================================
--- sys/conf/kern.pre.mk (revision 228799)
+++ sys/conf/kern.pre.mk (working copy)
@@ -29,15 +29,13 @@
.else
.if ${MACHINE_CPUARCH} == "powerpc"
_MINUS_O= -O # gcc miscompiles some code at -O2
+.elif ${MACHINE_CPUARCH} == "amd64" && ${CC:T:Mclang} != "clang"
+_MINUS_O= -O2 -frename-registers
.else
_MINUS_O= -O2
.endif
.endif
-.if ${MACHINE_CPUARCH} == "amd64"
-COPTFLAGS?=-O2 -frename-registers -pipe
-.else
COPTFLAGS?=${_MINUS_O} -pipe
-.endif
.if !empty(COPTFLAGS:M-O[23s]) && empty(COPTFLAGS:M-fno-strict-aliasing)
COPTFLAGS+= -fno-strict-aliasing
.endif
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