[llvm-commits] [llvm] r145913 - /llvm/trunk/docs/LangRef.html

[Eli Friedman]

On Mon, Dec 5, 2011 at 7:35 PM, Dan Gohman <gohman at apple.com> wrote:
> Author: djg
> Date: Mon Dec  5 21:35:58 2011
> New Revision: 145913
>
> URL: http://llvm.org/viewvc/llvm-project?rev=145913&view=rev
> Log:
> Fix a subtle semantic issue with poison values that came up in
> recent discussions. Poison can't make every value that depends on
> it act in maximally undefined ways, because the optimizer may still
> hoist code following the usual rules for undef. Make Poison invoke
> its full undefined behavior only when it reaches an instruction with
> externally visible side effects.
>
> Modified:
>    llvm/trunk/docs/LangRef.html
>
> Modified: llvm/trunk/docs/LangRef.html
> URL: http://llvm.org/viewvc/llvm-project/llvm/trunk/docs/LangRef.html?rev=145913&r1=145912&r2=145913&view=diff
> ==============================================================================
> --- llvm/trunk/docs/LangRef.html (original)
> +++ llvm/trunk/docs/LangRef.html Mon Dec  5 21:35:58 2011
> @@ -2512,10 +2512,9 @@
>  <div>
>
>  <p>Poison values are similar to <a href="#undefvalues">undef values</a>, however
> -   instead of representing an unspecified bit pattern, they represent the
> -   fact that an instruction or constant expression which cannot evoke side
> -   effects has nevertheless detected a condition which results in undefined
> -   behavior.</p>
> +   they also represent the fact that an instruction or constant expression which
> +   cannot evoke side effects has nevertheless detected a condition which results
> +   in undefined behavior.</p>
>
>  <p>There is currently no way of representing a poison value in the IR; they
>    only exist when produced by operations such as
> @@ -2572,22 +2571,21 @@
>
>  </ul>
>
> -<p>Whenever a poison value is generated, all values which depend on it evaluate
> -   to poison. If they have side effects, they evoke their side effects as if each
> -   operand with a poison value were undef. If they have externally-visible side
> -   effects, the behavior is undefined.</p>
> +<p>Poison Values have the same behavior as <a href="#undefvalues">undef values</a>,
> +   with the additional affect that any instruction which has a <i>dependence</i>
> +   on a poison value has undefined behavior.</p>
>
>  <p>Here are some examples:</p>
>
>  <pre class="doc_code">
>  entry:
>   %poison = sub nuw i32 0, 1           ; Results in a poison value.
> -  %still_poison = and i32 %poison, 0   ; Whereas (and i32 undef, 0) would return 0.
> +  %still_poison = and i32 %poison, 0   ; 0, but also poison.
>   %poison_yet_again = getelementptr i32* @h, i32 %still_poison
> -  store i32 0, i32* %poison_yet_again  ; undefined behavior
> +  store i32 0, i32* %poison_yet_again  ; memory at @h[0] is poisoned

This change can't be right... storing to a poisoned address has to be
undefined behavior in any sane model.

-Eli

> -  store i32 %poison, i32* @g           ; Poison value conceptually stored to memory.
> -  %poison2 = load i32* @g              ; Returns a poison value, not just undef.
> +  store i32 %poison, i32* @g           ; Poison value stored to memory.
> +  %poison2 = load i32* @g              ; Poison value loaded back from memory.
>
>   store volatile i32 %poison, i32* @g  ; External observation; undefined behavior.
>
> @@ -2626,8 +2624,8 @@
>   store volatile i32 0, i32* @g        ; This time, the instruction always depends
>                                        ; on the store in %end. Also, it is
>                                        ; control-equivalent to %end, so this is
> -                                       ; well-defined (again, ignoring earlier
> -                                       ; undefined behavior in this example).
> +                                       ; well-defined (ignoring earlier undefined
> +                                       ; behavior in this example).
>  </pre>
>
>  </div>
>
>
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