Date: Thu, 04 Oct 2012 22:02:20 +0200 From: Dimitry Andric <dimitry@andric.com> To: Andriy Gapon <avg@FreeBSD.org> Cc: freebsd-arch@FreeBSD.org Subject: Re: x86 boot code build Message-ID: <506DEB4C.5020508@andric.com> In-Reply-To: <506C385C.3020400@FreeBSD.org> References: <506C385C.3020400@FreeBSD.org>
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On 2012-10-03 15:06, Andriy Gapon wrote:
> Currently we produce "slightly" different binaries for x86 boot code depending
> whether MACHINE_CPUARCH is i386 or amd64. I think that there is no good reason
> for this, since in both cases we use exactly the same code and target the same
> classes of machines. In other words, the binaries should be interchangeable[*].
>
> The difference boils down to using -march=i386 on amd64 while i386 uses default
> compiler flags, which are equivalent to -march=i486 -mtune=generic.
Yes, I also noticed this inconsistency during some other work in
sys/boot, and I ended up with this diff in my backlog:
Index: sys/boot/i386/Makefile.inc
===================================================================
--- sys/boot/i386/Makefile.inc (revision 241194)
+++ sys/boot/i386/Makefile.inc (working copy)
@@ -5,12 +5,13 @@
BINDIR?= /boot
LOADER_ADDRESS?=0x200000
-CFLAGS+= -ffreestanding -mpreferred-stack-boundary=2 \
+CFLAGS+= -march=i386 -ffreestanding -mpreferred-stack-boundary=2 \
-mno-mmx -mno-3dnow -mno-sse -mno-sse2 -mno-sse3 -msoft-float
+NO_CPU_CFLAGS=
LDFLAGS+= -nostdlib
.if ${MACHINE_CPUARCH} == "amd64"
-CFLAGS+= -m32 -march=i386
+CFLAGS+= -m32
ACFLAGS+= -m32
LDFLAGS+= -m elf_i386_fbsd
AFLAGS+= --32
> If my analysis is correct, the only thing affected by the flags in the boot code
> is use of leave instruction when -Os is _not_ specified.
> For -march=i386 our gcc prefers using leave. For -march=i486 it thinks that
> movs+pops are faster than leave and so prefers to not use it. If -Os is
> specified, then leave is always used because it results in smaller machine code.
>
> So, as it is now, on amd64 we produce slightly smaller boot binaries where size
> doesn't matter. Where size really matters (-Os) we produce identical binaries.
>
> If we decide that it makes sense to converge i386 and amd64 boot build options,
> which should we pick?
Well, do we still officially support any real i386 machines? If so, we
should still use -march=i386 for the boot code. Otherwise, let's start
using -march=i486 explicitly. So like:
Index: sys/boot/i386/Makefile.inc
===================================================================
--- sys/boot/i386/Makefile.inc (revision 241194)
+++ sys/boot/i386/Makefile.inc (working copy)
@@ -5,12 +5,13 @@
BINDIR?= /boot
LOADER_ADDRESS?=0x200000
-CFLAGS+= -ffreestanding -mpreferred-stack-boundary=2 \
+CFLAGS+= -march=i486 -ffreestanding -mpreferred-stack-boundary=2 \
-mno-mmx -mno-3dnow -mno-sse -mno-sse2 -mno-sse3 -msoft-float
+NO_CPU_CFLAGS=
LDFLAGS+= -nostdlib
.if ${MACHINE_CPUARCH} == "amd64"
-CFLAGS+= -m32 -march=i386
+CFLAGS+= -m32
ACFLAGS+= -m32
LDFLAGS+= -m elf_i386_fbsd
AFLAGS+= --32
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