[PATCH] ARM: Fix build warning in do_alignment.

[PATCH] ARM: Fix build warning in do_alignment.

[Chanho Min]

Fix the following build warning:

arch/arm/mm/alignment.c: In function 'do_alignment':
arch/arm/mm/alignment.c:327:15: warning: 'offset.un' may be used
uninitialized in this function [-Wuninitialized]
arch/arm/mm/alignment.c:749:21: note: 'offset.un' was declared here

Signed-off-by: Chanho Min <chanho.min@lge.com>
---
 arch/arm/mm/alignment.c |    2 +-
 1 file changed, 1 insertion(+), 1 deletion(-)

diff --git a/arch/arm/mm/alignment.c b/arch/arm/mm/alignment.c
index 9107231..f83b38c 100644
--- a/arch/arm/mm/alignment.c
+++ b/arch/arm/mm/alignment.c
@@ -746,7 +746,7 @@ do_alignment_t32_to_handler(unsigned long *pinstr,
struct pt_regs *regs,
 static int
 do_alignment(unsigned long addr, unsigned int fsr, struct pt_regs *regs)
 {
-	union offset_union offset;
+	union offset_union offset = {0};
 	unsigned long instr = 0, instrptr;
 	int (*handler)(unsigned long addr, unsigned long instr, struct pt_regs *regs);
 	unsigned int type;
-- 
1.7.9.5

Re: [PATCH] ARM: Fix build warning in do_alignment.

[Russell King - ARM Linux]

On Mon, Sep 10, 2012 at 12:25:15PM +0900, Chanho Min wrote:
> Fix the following build warning:
> 
> arch/arm/mm/alignment.c: In function 'do_alignment':
> arch/arm/mm/alignment.c:327:15: warning: 'offset.un' may be used
> uninitialized in this function [-Wuninitialized]
> arch/arm/mm/alignment.c:749:21: note: 'offset.un' was declared here
> 
> Signed-off-by: Chanho Min <chanho.min@lge.com>

The compiler is being silly - notice that it is saying "may" not "is".
In this case, it will not be uninitialized prior to use.

What it's caused by is do_alignment_t32_to_handler(), and its assignment
through a pointer of this variable.  You can see that this is the cause
because the patch below fixes the warning.

It's all about whether gcc can prove that the variable is or is not
assigned.  It has one warning for the 'provably not assigned' case, and
another for the 'can't prove that it is always assigned' case.

Anyway, normally we don't fix warnings where the compiler is wrong (but
we encourage the compiler guys to fix the compiler instead.)

 arch/arm/mm/alignment.c |    6 +++---
 1 files changed, 3 insertions(+), 3 deletions(-)

diff --git a/arch/arm/mm/alignment.c b/arch/arm/mm/alignment.c
index 6d6790e..5ce7733 100644
--- a/arch/arm/mm/alignment.c
+++ b/arch/arm/mm/alignment.c
@@ -699,7 +699,6 @@ do_alignment_t32_to_handler(unsigned long *pinstr, struct pt_regs *regs,
 	unsigned long instr = *pinstr;
 	u16 tinst1 = (instr >> 16) & 0xffff;
 	u16 tinst2 = instr & 0xffff;
-	poffset->un = 0;
 
 	switch (tinst1 & 0xffe0) {
 	/* A6.3.5 Load/Store multiple */
@@ -854,9 +853,10 @@ do_alignment(unsigned long addr, unsigned int fsr, struct pt_regs *regs)
 		break;
 
 	case 0x08000000:	/* ldm or stm, or thumb-2 32bit instruction */
-		if (thumb2_32b)
+		if (thumb2_32b) {
+			offset.un = 0;
 			handler = do_alignment_t32_to_handler(&instr, regs, &offset);
-		else
+		} else
 			handler = do_alignment_ldmstm;
 		break;
 

Re: [PATCH] ARM: Fix build warning in do_alignment.

[Chanho Min]

> What it's caused by is do_alignment_t32_to_handler(), and its assignment
> through a pointer of this variable.  You can see that this is the cause
> because the patch below fixes the warning.
We couldn't fix the warning with your patch. the patch below fixes it instead.
IMHO, gcc can't seems to know that do_alignment_ldmstm never returns TYPE_LDST.
So It just warn that do_alignment_finish_ldst is called with the
uninitialized value.
I know gcc is not smart enough to see all the reasons. So, It is optional.
Also, I'm not sure these optional warning should be fixed.
Anyway, this is the last build warning in our whole source when using gcc-4.6.3

Thanks
Chanho,

diff --git a/arch/arm/mm/alignment.c b/arch/arm/mm/alignment.c
index 9107231..978db1f 100644
--- a/arch/arm/mm/alignment.c
+++ b/arch/arm/mm/alignment.c
@@ -856,8 +856,10 @@ do_alignment(unsigned long addr, unsigned int
fsr, struct pt_regs *regs)
 	case 0x08000000:	/* ldm or stm, or thumb-2 32bit instruction */
 		if (thumb2_32b)
 			handler = do_alignment_t32_to_handler(&instr, regs, &offset);
-		else
+		else {
+			offset.un = 0;
 			handler = do_alignment_ldmstm;
+		}
 		break;

 	default: