[PATCH] sparse: fix bogus shift too big warning when rhs is a variable

[PATCH] sparse: fix bogus shift too big warning when rhs is a variable

[Sam Ravnborg]

From 3e742071e931669108f55b4aae2fb074d1c029cc Mon Sep 17 00:00:00 2001
From: Sam Ravnborg <sam@ravnborg.org>
Date: Mon, 16 Jun 2014 19:02:05 +0200
Subject: [PATCH] sparse: fix bogus shift too big warning when rhs is a
 variable

sparse warned about "shift too big" if the rhs was a variable.
As sparse usually cannot deduct the value of a variable
silence these warnings.

Before this patch following code snippet would generate a warning:

static int doshift(int foo)
{
	return 1 << foo;
}

sparse did not know the value of 'foo' so reported a warning.
Ignore the warning expect in the cases where the rhs of the shift
is a constant.

Include a few simple tests for left-shift and right-shift.

Reported-by: Dan Carpenter <dan.carpenter@oracle.com>
Signed-off-by: Sam Ravnborg <sam@ravnborg.org>
---
 expand.c                 |  2 +-
 validation/left-shift.c  | 31 +++++++++++++++++++++++++++++++
 validation/right-shift.c | 31 +++++++++++++++++++++++++++++++
 3 files changed, 63 insertions(+), 1 deletion(-)
 create mode 100644 validation/left-shift.c
 create mode 100644 validation/right-shift.c

diff --git a/expand.c b/expand.c
index 0f6720c..ce955d3 100644
--- a/expand.c
+++ b/expand.c
@@ -187,7 +187,7 @@ static int simplify_int_binop(struct expression *expr, struct symbol *ctype)
 		return 0;
 	r = right->value;
 	if (expr->op == SPECIAL_LEFTSHIFT || expr->op == SPECIAL_RIGHTSHIFT) {
-		if (r >= ctype->bit_size) {
+		if (right->flags & Int_const_expr && r >= ctype->bit_size) {
 			if (conservative)
 				return 0;
 			r = check_shift_count(expr, ctype, r);
diff --git a/validation/left-shift.c b/validation/left-shift.c
new file mode 100644
index 0000000..e8d929b
--- /dev/null
+++ b/validation/left-shift.c
@@ -0,0 +1,31 @@
+static unsigned int leftshift1(unsigned int value, unsigned int shift)
+{
+        return 1 << 1000;
+}
+
+static unsigned int leftshift2(unsigned int value, unsigned int shift)
+{
+        return value << 1000;
+}
+
+static unsigned int leftshift3(unsigned int value, unsigned int shift)
+{
+
+        return 1 << value;
+}
+
+static unsigned int leftshift4(unsigned int value, unsigned int shift)
+{
+
+        return value << shift;
+}
+
+/*
+ * check-name: Left shift - shift too big
+ *
+ * check-error-start
+left-shift.c:3:18: warning: shift too big (1000) for type int
+left-shift.c:8:22: warning: shift too big (1000) for type unsigned int
+ * check-error-end
+ */
+
diff --git a/validation/right-shift.c b/validation/right-shift.c
new file mode 100644
index 0000000..9be6006
--- /dev/null
+++ b/validation/right-shift.c
@@ -0,0 +1,31 @@
+static unsigned int rightshift1(unsigned int value, unsigned int shift)
+{
+        return 1 >> 1000;
+}
+
+static unsigned int rightshift2(unsigned int value, unsigned int shift)
+{
+        return value >> 1000;
+}
+
+static unsigned int rightshift3(unsigned int value, unsigned int shift)
+{
+
+        return 1 >> value;
+}
+
+static unsigned int rightshift4(unsigned int value, unsigned int shift)
+{
+
+        return value >> shift;
+}
+
+/*
+ * check-name: Right shift - shift too big
+ *
+ * check-error-start
+right-shift.c:3:18: warning: shift too big (1000) for type int
+right-shift.c:8:22: warning: shift too big (1000) for type unsigned int
+ * check-error-end
+ */
+
-- 
1.9.3

Re: [PATCH] sparse: fix bogus shift too big warning when rhs is a variable

[josh]

On Mon, Jun 16, 2014 at 07:10:06PM +0200, Sam Ravnborg wrote:
> From 3e742071e931669108f55b4aae2fb074d1c029cc Mon Sep 17 00:00:00 2001
> From: Sam Ravnborg <sam@ravnborg.org>
> Date: Mon, 16 Jun 2014 19:02:05 +0200
> Subject: [PATCH] sparse: fix bogus shift too big warning when rhs is a
>  variable
> 
> sparse warned about "shift too big" if the rhs was a variable.
> As sparse usually cannot deduct the value of a variable
> silence these warnings.
> 
> Before this patch following code snippet would generate a warning:
> 
> static int doshift(int foo)
> {
> 	return 1 << foo;
> }
> 
> sparse did not know the value of 'foo' so reported a warning.
> Ignore the warning expect in the cases where the rhs of the shift

s/expect/except/

> is a constant.
> 
> Include a few simple tests for left-shift and right-shift.
> 
> Reported-by: Dan Carpenter <dan.carpenter@oracle.com>
> Signed-off-by: Sam Ravnborg <sam@ravnborg.org>

Reviewed-by: Josh Triplett <josh@joshtriplett.org>

Thanks for fixing this!

> ---
>  expand.c                 |  2 +-
>  validation/left-shift.c  | 31 +++++++++++++++++++++++++++++++
>  validation/right-shift.c | 31 +++++++++++++++++++++++++++++++
>  3 files changed, 63 insertions(+), 1 deletion(-)
>  create mode 100644 validation/left-shift.c
>  create mode 100644 validation/right-shift.c
> 
> diff --git a/expand.c b/expand.c
> index 0f6720c..ce955d3 100644
> --- a/expand.c
> +++ b/expand.c
> @@ -187,7 +187,7 @@ static int simplify_int_binop(struct expression *expr, struct symbol *ctype)
>  		return 0;
>  	r = right->value;
>  	if (expr->op == SPECIAL_LEFTSHIFT || expr->op == SPECIAL_RIGHTSHIFT) {
> -		if (r >= ctype->bit_size) {
> +		if (right->flags & Int_const_expr && r >= ctype->bit_size) {
>  			if (conservative)
>  				return 0;
>  			r = check_shift_count(expr, ctype, r);
> diff --git a/validation/left-shift.c b/validation/left-shift.c
> new file mode 100644
> index 0000000..e8d929b
> --- /dev/null
> +++ b/validation/left-shift.c
> @@ -0,0 +1,31 @@
> +static unsigned int leftshift1(unsigned int value, unsigned int shift)
> +{
> +        return 1 << 1000;
> +}
> +
> +static unsigned int leftshift2(unsigned int value, unsigned int shift)
> +{
> +        return value << 1000;
> +}
> +
> +static unsigned int leftshift3(unsigned int value, unsigned int shift)
> +{
> +
> +        return 1 << value;
> +}
> +
> +static unsigned int leftshift4(unsigned int value, unsigned int shift)
> +{
> +
> +        return value << shift;
> +}
> +
> +/*
> + * check-name: Left shift - shift too big
> + *
> + * check-error-start
> +left-shift.c:3:18: warning: shift too big (1000) for type int
> +left-shift.c:8:22: warning: shift too big (1000) for type unsigned int
> + * check-error-end
> + */
> +
> diff --git a/validation/right-shift.c b/validation/right-shift.c
> new file mode 100644
> index 0000000..9be6006
> --- /dev/null
> +++ b/validation/right-shift.c
> @@ -0,0 +1,31 @@
> +static unsigned int rightshift1(unsigned int value, unsigned int shift)
> +{
> +        return 1 >> 1000;
> +}
> +
> +static unsigned int rightshift2(unsigned int value, unsigned int shift)
> +{
> +        return value >> 1000;
> +}
> +
> +static unsigned int rightshift3(unsigned int value, unsigned int shift)
> +{
> +
> +        return 1 >> value;
> +}
> +
> +static unsigned int rightshift4(unsigned int value, unsigned int shift)
> +{
> +
> +        return value >> shift;
> +}
> +
> +/*
> + * check-name: Right shift - shift too big
> + *
> + * check-error-start
> +right-shift.c:3:18: warning: shift too big (1000) for type int
> +right-shift.c:8:22: warning: shift too big (1000) for type unsigned int
> + * check-error-end
> + */
> +
> -- 
> 1.9.3
> 
> --
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Re: [PATCH] sparse: fix bogus shift too big warning when rhs is a variable

[Christopher Li]

Still catching up my emails.

On Mon, Jun 16, 2014 at 10:10 AM, Sam Ravnborg <sam@ravnborg.org> wrote:
> sparse warned about "shift too big" if the rhs was a variable.
> As sparse usually cannot deduct the value of a variable
> silence these warnings.

That is very strange. It should not happen.
>
> Before this patch following code snippet would generate a warning:
>
> static int doshift(int foo)
> {
>         return 1 << foo;
> }
>
> sparse did not know the value of 'foo' so reported a warning.
> Ignore the warning expect in the cases where the rhs of the shift
> is a constant.

That is strange. I can't duplicate the bug with your test case:
With tip of chrisl repository,
I get the following error with your test case:
./sparse /tmp/b.c
/tmp/b.c:3:18: warning: shift too big (1000) for type int
/tmp/b.c:8:22: warning: shift too big (1000) for type unsigned int


>
> diff --git a/expand.c b/expand.c
> index 0f6720c..ce955d3 100644
> --- a/expand.c
> +++ b/expand.c
> @@ -187,7 +187,7 @@ static int simplify_int_binop(struct expression *expr, struct symbol *ctype)
>                 return 0;
>         r = right->value;
>         if (expr->op == SPECIAL_LEFTSHIFT || expr->op == SPECIAL_RIGHTSHIFT) {
> -               if (r >= ctype->bit_size) {
> +               if (right->flags & Int_const_expr && r >= ctype->bit_size) {


This does not explain why it fix the bug. In simplify_int_binop()
There is the code check for this is constant expr on rhs. The non
constant value should just bail out early.

    if (right->type != EXPR_VALUE)
        return 0;

That is right before your change. Plus I can't duplicate the bug myself.

I suspect there is something else going on. Any way to help me reproduce
the bug?

I am using gcc 4.8.2 on x86_64.

Chris

Re: [PATCH] sparse: fix bogus shift too big warning when rhs is a variable

[Christopher Li]

On Sat, Jun 28, 2014 at 10:32 AM, Christopher Li <sparse@chrisli.org> wrote:

>
> That is right before your change. Plus I can't duplicate the bug myself.
>
> I suspect there is something else going on. Any way to help me reproduce
> the bug?

Never mind, I just saw the other email thread discussing this bug.
I will try the test case provide there.

Chris

Re: [PATCH] sparse: fix bogus shift too big warning when rhs is a variable

[Christopher Li]

On Sat, Jun 28, 2014 at 10:37 AM, Christopher Li <sparse@chrisli.org> wrote:
>
> Never mind, I just saw the other email thread discussing this bug.
> I will try the test case provide there.
>


Ok, I just reply to the other email.
The bug is in the kernel source, a valid complain.
Sparse is doing the right thing here. Not a sparse bug, it is feature.

Chris