Re: [PATCH 2/4] msm: scm: Fix improper register assignment

From: Saravana Kannan
Date: Sat Feb 26 2011 - 00:09:19 EST

On 02/25/2011 05:23 AM, Will Deacon wrote:
On Thu, 2011-02-24 at 18:44 +0000, Stephen Boyd wrote:
Assign the registers used in the inline assembly immediately
before the inline assembly block. This ensures the compiler
doesn't optimize away dead register assignments when it

Signed-off-by: Stephen Boyd<sboyd@xxxxxxxxxxxxxx>
arch/arm/mach-msm/scm.c | 7 +++++--
1 files changed, 5 insertions(+), 2 deletions(-)

diff --git a/arch/arm/mach-msm/scm.c b/arch/arm/mach-msm/scm.c
index ba57b5a..5eddf54 100644
--- a/arch/arm/mach-msm/scm.c
+++ b/arch/arm/mach-msm/scm.c
@@ -264,13 +264,16 @@ u32 scm_get_version(void)
int context_id;
static u32 version = -1;
- register u32 r0 asm("r0") = 0x1<< 8;
- register u32 r1 asm("r1") = (u32)&context_id;
+ register u32 r0 asm("r0");
+ register u32 r1 asm("r1");

if (version != -1)
return version;

+ r0 = 0x1<< 8;
+ r1 = (u32)&context_id;
asm volatile(
__asmeq("%0", "r1")
__asmeq("%1", "r0")

Whoa, have you seen the compiler `optimise' the original assignments
away? Since there is a use in the asm block, the definition shouldn't
be omitted. What toolchain are you using?

Yeah, Stephen and I spent quite a bit of time discussing this and experimenting to figure out what the heck GCC was doing. But it kept optimizing the fake code we put in trying to force GCC to use a specific register.

My hypothesis at this point is that the "register xx asm("rx")" declarations are just for giving a symbolic name to refer to the specific register in C code. I doesn't tell GCC to reserve away the register and make sure the value is preserved. And the assignments to these said variables seem to translate to a pure "mov rx, 5" kinda instruction with no further preservation of rx either.

That's the only hypothesis I/we could come up with as to how this got optimized away.

I would be great if someone explains the exact meaning of these "register asm" declarations and the assignments in C code.


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