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Re: [RFC] Optimize swab operations on mips_r2 cpu

To: "Kevin D. Kissell" <kevink@mips.com>
Subject: Re: [RFC] Optimize swab operations on mips_r2 cpu
From: Franck <vagabon.xyz@gmail.com>
Date: Fri, 27 Jan 2006 15:44:31 +0100
Cc: Nigel Stephens <nigel@mips.com>, linux-mips@linux-mips.org
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2006/1/27, Kevin D. Kissell <kevink@mips.com>:
> > > Configuration hacks that are specific to a single core create cruft and
> > > maintenence problems.  As I said yesterday, I think we'd be much better
> > > off to have a CONFIG_CPU_MIPS_SMALL or some such option
> > > that could cause -Os to be used, allow branch-likelies, etc. The 
> > > optimizations
> > > under discussion aren't at all specific to the 4KSd,
> >
> > no some are. As we said previously:
> >
> >         1/ sizeof(vmlinux-mips32r2-Os) > sizeof(vmlinux-4ksd-Os)
> >         2/ with -march=4ksd can do (slightly) better optimizations.
>
> This is very possibly due to the compiler knowing about the SmartMIPS
> scaled, indexed load instructions, which were added to improve virtual
> machine performance, but which also save on address calculation instructions.
> If -march=mips32r2 combined with -msmartmips and -Os don't produce
> pretty much the same result as -march=4ksd, I'd be interested in knowing
> why.

well, Nigel gave us some answers here, no ? I'm not sure it's a good
idea to use -march=mips32r2 whatever the mips32r2 cpu is used. Even if
the gain is small, we should use a specific cpu description. If the
configuration makes it hard, it should be reworked, isn't it ?

> Regardless, if this is what's going on, there really is no other core
> in production today that will run that code.  But that doesn't mean
> that there won't be others in the future.
>
> All I'm really trying to do here is to get away from core-specific config
> cruft.  If the best result under-the-hood for 4KSd is obtained by using
> -march=4ksd, that's what people should get, but I'd rather that users
> and maintainers saw that as a choice of MIPS32R2+SmartMIPS rather
> than a choice of 4KSd as a one-off.
>

Ok what about this patch. It allows to choose a specific cpu when
MIPS32R2 architecture is selected ?

diff --git a/arch/mips/Kconfig b/arch/mips/Kconfig
index 8b9002a..a6d168c 100644
--- a/arch/mips/Kconfig
+++ b/arch/mips/Kconfig
@@ -1290,6 +1302,30 @@ config SYS_HAS_CPU_RM9000
 config SYS_HAS_CPU_SB1
        bool

+#
+# CPU model tuning.
+#
+choice
+       prompt "CPU model"
+       default CPU_GENERIC
+       help
+         When configuring kernel building to MIPS32 architecture,
+         whatever the release number, it can be interesting to narrow
+         your choice for speed and size optimizations.
+
+config CPU_GENERIC
+       bool "Generic"
+       help
+         It's safe to select this if your CPU model doesn't appear in the list.
+
+config CPU_4KSD
+       bool "4KSD"
+       depends on SYS_HAS_CPU_MIPS32_R2
+       help
+         MIPS Technologies 4KSd-series processors.
+
+endchoice
+
 endmenu

 #
diff --git a/arch/mips/Makefile b/arch/mips/Makefile
index bc99c2d..c11df11 100644
--- a/arch/mips/Makefile
+++ b/arch/mips/Makefile
@@ -165,82 +167,92 @@ echo $$gcc_abi $$gcc_opt$$gcc_cpu $$gcc_
 #
 # CPU-dependent compiler/assembler options for optimization.
 #
-cflags-$(CONFIG_CPU_R3000)     += \
+
+__cflags-$(CONFIG_CPU_R3000)   += \
                        $(call set_gccflags,r3000,mips1,r3000,mips1,mips1)

-cflags-$(CONFIG_CPU_TX39XX)    += \
+__cflags-$(CONFIG_CPU_TX39XX)  += \
                        $(call set_gccflags,r3900,mips1,r3000,mips1,mips1)

-cflags-$(CONFIG_CPU_R6000)     += \
+__cflags-$(CONFIG_CPU_R6000)   += \
                        $(call set_gccflags,r6000,mips2,r6000,mips2,mips2) \
                        -Wa,--trap

-cflags-$(CONFIG_CPU_R4300)     += \
+__cflags-$(CONFIG_CPU_R4300)   += \
                        $(call set_gccflags,r4300,mips3,r4300,mips3,mips2) \
                        -Wa,--trap

-cflags-$(CONFIG_CPU_VR41XX)    += \
+__cflags-$(CONFIG_CPU_VR41XX)  += \
                        $(call set_gccflags,r4100,mips3,r4600,mips3,mips2) \
                        -Wa,--trap

-cflags-$(CONFIG_CPU_R4X00)     += \
+__cflags-$(CONFIG_CPU_R4X00)   += \
                        $(call set_gccflags,r4600,mips3,r4600,mips3,mips2) \
                        -Wa,--trap

-cflags-$(CONFIG_CPU_TX49XX)    += \
+__cflags-$(CONFIG_CPU_TX49XX)  += \
                        $(call set_gccflags,r4600,mips3,r4600,mips3,mips2)  \
                        -Wa,--trap

-cflags-$(CONFIG_CPU_MIPS32_R1) += \
-                       $(call set_gccflags,mips32,mips32,r4600,mips3,mips2) \
-                       -Wa,--trap
-
-cflags-$(CONFIG_CPU_MIPS32_R2) += \
-                       $(call 
set_gccflags,mips32r2,mips32r2,r4600,mips3,mips2) \
-                       -Wa,--trap
-
-cflags-$(CONFIG_CPU_MIPS64_R1) += \
-                       $(call set_gccflags,mips64,mips64,r4600,mips3,mips2) \
-                       -Wa,--trap
-
-cflags-$(CONFIG_CPU_MIPS64_R2) += \
-                       $(call 
set_gccflags,mips64r2,mips64r2,r4600,mips3,mips2) \
-                       -Wa,--trap
-
-cflags-$(CONFIG_CPU_R5000)     += \
+__cflags-$(CONFIG_CPU_R5000)   += \
                        $(call set_gccflags,r5000,mips4,r5000,mips4,mips2) \
                        -Wa,--trap

-cflags-$(CONFIG_CPU_R5432)     += \
+__cflags-$(CONFIG_CPU_R5432)   += \
                        $(call set_gccflags,r5400,mips4,r5000,mips4,mips2) \
                        -Wa,--trap

-cflags-$(CONFIG_CPU_NEVADA)    += \
+__cflags-$(CONFIG_CPU_NEVADA)  += \
                        $(call set_gccflags,rm5200,mips4,r5000,mips4,mips2) \
                        -Wa,--trap
 #                      $(call cc-option,-mmad)

-cflags-$(CONFIG_CPU_RM7000)    += \
+__cflags-$(CONFIG_CPU_RM7000)  += \
                        $(call set_gccflags,rm7000,mips4,r5000,mips4,mips2) \
                        -Wa,--trap

-cflags-$(CONFIG_CPU_RM9000)    += \
+__cflags-$(CONFIG_CPU_RM9000)  += \
                        $(call set_gccflags,rm9000,mips4,r5000,mips4,mips2) \
                        -Wa,--trap


-cflags-$(CONFIG_CPU_SB1)       += \
+__cflags-$(CONFIG_CPU_SB1)     += \
                        $(call set_gccflags,sb1,mips64,r5000,mips4,mips2) \
                        -Wa,--trap

-cflags-$(CONFIG_CPU_R8000)     += \
+__cflags-$(CONFIG_CPU_R8000)   += \
                        $(call set_gccflags,r8000,mips4,r8000,mips4,mips2) \
                        -Wa,--trap

-cflags-$(CONFIG_CPU_R10000)    += \
+__cflags-$(CONFIG_CPU_R10000)  += \
                        $(call set_gccflags,r10000,mips4,r8000,mips4,mips2) \
                        -Wa,--trap

+__cflags-$(CONFIG_CPU_4KSD)    += \
+                       $(call set_gccflags,4ksd,mips32r2,4kec,mips32r2) \
+                       -Wa,--trap
+
+# Fallback options for MIPS32 architectures
+ifndef __cflags-y
+__cflags-$(CONFIG_CPU_MIPS32_R1)       += \
+                       $(call set_gccflags,mips32,mips32,r4600,mips3,mips2) \
+                       -Wa,--trap
+
+__cflags-$(CONFIG_CPU_MIPS32_R2)       += \
+                       $(call 
set_gccflags,mips32r2,mips32r2,r4600,mips3,mips2) \
+                       -Wa,--trap
+
+__cflags-$(CONFIG_CPU_MIPS64_R1)       += \
+                       $(call set_gccflags,mips64,mips64,r4600,mips3,mips2) \
+                       -Wa,--trap
+
+__cflags-$(CONFIG_CPU_MIPS64_R2)       += \
+                       $(call 
set_gccflags,mips64r2,mips64r2,r4600,mips3,mips2) \
+                       -Wa,--trap
+endif
+
+cflags-y       += $(__cflags-y)
+
 ifdef CONFIG_CPU_SB1
 ifdef CONFIG_SB1_PASS_1_WORKAROUNDS
 MODFLAGS       += -msb1-pass1-workarounds

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