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SubjectRe: ftrace breaks sparc64 build

On Mon, 5 Jan 2009, Sam Ravnborg wrote:

> With an allmodconfig build on sparc and sparc64 I started
> to see warnings that become propagated to errors by -Werror.
>
> Example:
> CC arch/sparc/kernel/ldc.o
> arch/sparc/kernel/ldc.c: In function `process_control_frame':
> arch/sparc/kernel/ldc.c:627: warning: 'vap' might be used uninitialized in this function
>
>
> The code in question looks like this:
> static int process_ver_nack(struct ldc_channel *lp, struct ldc_version *vp)
> {
> struct ldc_version *vap;
>
> if ((vp->major == 0 && vp->minor == 0) ||
> !(vap = find_by_major(vp->major))) {
> return ldc_abort(lp);
> } else {
> struct ldc_packet *p;
> unsigned long new_tail;
>
> p = handshake_compose_ctrl(lp, LDC_INFO, LDC_VERS,
> vap, sizeof(*vap),
> &new_tail);
> if (p)
> return send_tx_packet(lp, p, new_tail);
> else
> return ldc_abort(lp);
> }
> }
>
> The else part will never be executed whitout assigning vap,
> and this code do not emit warnings in the normal case.
> [I am well aware that we recommend to move the assignment
> out of the if () - but this code worked as is before].
>
> This code gets expanded to:
>
> static int process_ver_nack(struct ldc_channel *lp, struct ldc_version *vp)
> {
> struct ldc_version *vap;
>
> if (__builtin_constant_p(((vp->major == 0 && vp->minor == 0) || !(vap = find_by_major(vp->major)))) ?
> !!((vp->major == 0 && vp->minor == 0) || !(vap = find_by_major(vp->major))) :
> ({
> int ______r;
> static struct ftrace_branch_data ______f = { .func = __func__, .file = "arch/sparc/kernel/ldc.c", .line = 630, };
> ______r = !!((vp->major == 0 && vp->minor == 0) || !(vap = find_by_major(vp->major)));
> if (______r)
> ______f.hit++;
> else
> ______f.miss++; ______r;
> })) {
>
> return ldc_abort(lp);
> } else {
> struct ldc_packet *p;
> unsigned long new_tail;
>
> p = handshake_compose_ctrl(lp, 0x01, 0x01, vap, sizeof(*vap), &new_tail);
> if (__builtin_constant_p((p)) ? !!(p) : ({
> int ______r;
> static struct ftrace_branch_data ______f = { .func = __func__, .file = "arch/sparc/kernel/ldc.c", .line = 639, };
> ______r = !!(p);
> if (______r)
> ______f.hit++;
> else ______f.miss++;
> ______r;
> }))
> return send_tx_packet(lp, p, new_tail);
> else
> return ldc_abort(lp);
> }
> }
> I have inserted newlines + tabs and removed a few __attribute__()
> to keep line lengths to a sensible level.
>
> My head started to spin with a dangerous speed trying to figure out
> the code snippet above.

My head spun a little by figuring out that vap is initialized in
the original code.

Honestly, that code is a little obfuscated, and would be better to write
it as:

if (vp->major == 0 && vp->minor=0)
return ldc_abort(lp);
vap = find_by_major(vp->major);
if (!vap)
return ldc_abort(lp);
[...]

This is much easier to read and we can remove the else statement
altogether. And I bet the warning will go away if we did it this way.

-- Steve


>
> On top of this some inlining occurs which is why gcc point at another
> function name.
>
>
> This is with following gcc version:
>
> sparc64-unknown-linux-gnu-gcc (GCC) 3.4.5
> Copyright (C) 2004 Free Software Foundation, Inc.
> This is free software; see the source for copying conditions. There is NO
> warranty; not even for MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.
>
> Build using crosstool.
>
> Is this a known issue?
>
> Any recommendations?
>
> Sam
>
>


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