[Bug middle-end/4210] should not warning with dead code

[Bug middle-end/4210] should not warning with dead code

[pinskia at gcc dot gnu dot org]

------- Comment #12 from pinskia at gcc dot gnu dot org  2006-03-01 19:02 -------
*** Bug 26516 has been marked as a duplicate of this bug. ***


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pinskia at gcc dot gnu dot org changed:

           What    |Removed                     |Added
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                 CC|                            |mnmoran at bellsouth dot net


http://gcc.gnu.org/bugzilla/show_bug.cgi?id=4210

[Bug middle-end/4210] should not warning with dead code

[joseph at codesourcery dot com]

------- Comment #13 from joseph at codesourcery dot com  2006-03-01 23:01 -------
Subject: Re:  should not warning with dead code

A workaround is to use ? : and statement expressions instead of "if".  
This way, the front-end setting of skip_evaluation disables these 
warnings.  (skip_evaluation can't be set for if (0) because you can jump 
into if (0), whereas jumps into statement expressions are not permitted.  
Thus in general you need to parse the whole function body to tell if the 
if (0) is dead.)


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http://gcc.gnu.org/bugzilla/show_bug.cgi?id=4210

[Bug middle-end/4210] should not warning with dead code

[bangerth at dealii dot org]

------- Comment #14 from bangerth at dealii dot org  2006-03-01 23:05 -------
But that's
a) clearly a kludge,
b) may not help in the future if our optimizers become more elaborate
c) doesn't work with code where the code guarded by the if(0) is more
   than a single statement.
It would definitely be better to have this fixed in the right place...

W.


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http://gcc.gnu.org/bugzilla/show_bug.cgi?id=4210

[Bug middle-end/4210] should not warning with dead code

[joseph at codesourcery dot com]

------- Comment #15 from joseph at codesourcery dot com  2006-03-01 23:22 -------
Subject: Re:  should not warning with dead code

On Wed, 1 Mar 2006, bangerth at dealii dot org wrote:

> c) doesn't work with code where the code guarded by the if(0) is more
>    than a single statement.

It does; I've used it to eliminate all these warnings in glibc's soft-fp 
code.  Use statement expressions, i.e. surround the whole if body with ({ 
}).


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http://gcc.gnu.org/bugzilla/show_bug.cgi?id=4210

[Bug middle-end/4210] should not warning with dead code

[bangerth at dealii dot org]

------- Comment #16 from bangerth at dealii dot org  2006-03-01 23:25 -------

> It does; I've used it to eliminate all these warnings in glibc's soft-fp 
> code.  Use statement expressions, i.e. surround the whole if body with ({ 
> }).

Ugh. Do we really want to advocate serious code obfuscation to avoid warnings?
W.


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http://gcc.gnu.org/bugzilla/show_bug.cgi?id=4210

[Bug middle-end/4210] should not warning with dead code

[mattias at virtutech dot se]

------- Comment #17 from mattias at virtutech dot se  2006-03-02 09:22 -------
We have resorted to case-by-case workarounds, usually a cast which would have
been an identity operation had the condition been true. That is,

if (sizeof x == 8)
        return x << 32 | x;

would have its second line mutated to

        return (unsigned long long)x << 32 | x;

The cast is always a no-op unless it occurs in dead code.


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http://gcc.gnu.org/bugzilla/show_bug.cgi?id=4210

[Bug middle-end/4210] should not warning with dead code

[pinskia at gcc dot gnu dot org]

------- Comment #18 from pinskia at gcc dot gnu dot org  2006-06-20 19:05 -------
*** Bug 28106 has been marked as a duplicate of this bug. ***


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pinskia at gcc dot gnu dot org changed:

           What    |Removed                     |Added
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                 CC|                            |eggert at gnu dot org


http://gcc.gnu.org/bugzilla/show_bug.cgi?id=4210

[Bug middle-end/4210] should not warning with dead code

[pinskia at gcc dot gnu dot org]

------- Comment #19 from pinskia at gcc dot gnu dot org  2007-01-01 19:03 -------
*** Bug 30343 has been marked as a duplicate of this bug. ***


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pinskia at gcc dot gnu dot org changed:

           What    |Removed                     |Added
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                 CC|                            |gjasny at web dot de


http://gcc.gnu.org/bugzilla/show_bug.cgi?id=4210

[Bug middle-end/4210] should not warning with dead code

[pinskia at gcc dot gnu dot org]

------- Comment #20 from pinskia at gcc dot gnu dot org  2009-05-12 14:49 -------
*** Bug 40114 has been marked as a duplicate of this bug. ***


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pinskia at gcc dot gnu dot org changed:

           What    |Removed                     |Added
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                 CC|                            |gowen at innovative-
                   |                            |technology dot co dot uk


http://gcc.gnu.org/bugzilla/show_bug.cgi?id=4210

[Bug middle-end/4210] should not warning with dead code

[manu at gcc dot gnu dot org]

------- Comment #21 from manu at gcc dot gnu dot org  2010-07-06 17:24 -------
*** Bug 44842 has been marked as a duplicate of this bug. ***


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manu at gcc dot gnu dot org changed:

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                 CC|                            |vincent at vinc17 dot org


http://gcc.gnu.org/bugzilla/show_bug.cgi?id=4210

[Bug middle-end/4210] should not warning with dead code

[manu at gcc dot gnu dot org]

------- Comment #22 from manu at gcc dot gnu dot org  2010-07-06 17:28 -------
The way Clang gets this right is to perform some very-fast bitmap common
constant propagation in the FE. I personally think this would be helpful if
implemented correctly, even if it slows down the FE a little. But do not wait
for a fix soon because this is not trivial to fix and no one has both the
desire and the free time to work on fixing this.


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http://gcc.gnu.org/bugzilla/show_bug.cgi?id=4210

[Bug middle-end/4210] should not warning with dead code

[vincent-gcc at vinc17 dot net]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=4210

--- Comment #33 from Vincent Lefèvre <vincent-gcc at vinc17 dot net> ---
(In reply to Niels Möller from comment #32)
> 4. I also wonder what happens if, for some reason, a constant invalid shift
> count is passed through all the way to code generation? Most architectures
> would represent a constant shift count for a 32-bit value as 5-bit field in
> the opcode, and then the invalid shift counts aren't representable at all.
> Will gcc silently ignore higher bits,

That's undefined behavior, so that GCC can do whatever it wants for the
generated code.

> or is it an internal compiler error,

This would not be acceptable.

[Bug middle-end/4210] should not warning with dead code

[nisse at lysator dot liu.se]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=4210

--- Comment #32 from Niels Möller <nisse at lysator dot liu.se> ---
I've checked out the gcc sources, to see if I can understand how to move the
warning around. The example input I'm looking at now is  

unsigned 
shift_dead (unsigned x)
{
  if (0)
    return x >> 32;
  else
    return x >> 1;
}

unsigned 
shift_invalid (unsigned x)
{
  return x >> 32;
}

where I'd like gcc -Wall to warn for the second function, but not from the
first (currently, it warns for both). The warning is emitted from
build_binary_op, in gcc/c/c-typeck.c, close to line 11880. Deleting it there
silences the warning for *both* functions above.

I have a few questions. Keep in mind that I only have a very vague
understanding of the different phases in gcc, so any guidance is appreciated. 

1. There's similar code in c_fully_fold_internal, in gcc/c/c-fold.c, close to
line 400. But that code does *not* emit any warning for the example above,
which surprised me a bit. Maybe that's only for the case that both operands to
the shift operator are constants?

2. More importantly, if the warning is deleted from build_binary_op, we need to
add a check for this case, and an appropriate warning, somewhere later in the
compilation process. It has to be done after dead code is identified, i.e., in
a phase processing only non-dead code. And preferably as early as possibly,
when we're still working close to the parse-tree representation. Is there such
a place? Some other functions traversing the tree are c_gimplify_expr
(gcc/c-family/c-gimplify.c) and verify_tree (gcc/c-family/c-common.c), are any
of those called after elimination of dead code?

3. Alternatively, if there's no place after dead code elimination where the
parse tree is still easily available, a different alternative could be to leave
the check for invalid shift counts in c-typeck.c, but instead of emitting a
warning, construct a special tree object representing an expression with
invalid/undefined behavior, and any meta data needed to emit a warning or error
to describe it? Then emission of the warning could be postponed to later, say,
close to the conversion to SSA form?

4. I also wonder what happens if, for some reason, a constant invalid shift
count is passed through all the way to code generation? Most architectures
would represent a constant shift count for a 32-bit value as 5-bit field in the
opcode, and then the invalid shift counts aren't representable at all. Will gcc
silently ignore higher bits, or is it an internal compiler error, or would it
make sense to produce a friendly warning at this late stage of the compilation?

[Bug middle-end/4210] should not warning with dead code

[nisse at lysator dot liu.se]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=4210

Niels Möller <nisse at lysator dot liu.se> changed:

           What    |Removed                     |Added
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                 CC|                            |nisse at lysator dot liu.se

--- Comment #31 from Niels Möller <nisse at lysator dot liu.se> ---
*** Bug 94773 has been marked as a duplicate of this bug. ***

[Bug middle-end/4210] should not warning with dead code

[pinskia at gcc dot gnu.org]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=4210

Andrew Pinski <pinskia at gcc dot gnu.org> changed:

           What    |Removed                     |Added
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                 CC|                            |brooks at gcc dot gnu.org

--- Comment #24 from Andrew Pinski <pinskia at gcc dot gnu.org> ---
*** Bug 62074 has been marked as a duplicate of this bug. ***

[Bug middle-end/4210] should not warning with dead code

[pinskia at gcc dot gnu.org]

http://gcc.gnu.org/bugzilla/show_bug.cgi?id=4210

Andrew Pinski <pinskia at gcc dot gnu.org> changed:

           What    |Removed                     |Added
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                 CC|                            |xzfcpw+gcc at gmail dot com

--- Comment #23 from Andrew Pinski <pinskia at gcc dot gnu.org> ---
*** Bug 60513 has been marked as a duplicate of this bug. ***