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Fix a subtle semantic issue with poison values that came up in

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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.

llvm-svn: 145913
This commit is contained in:
Dan Gohman 2011-12-06 03:35:58 +00:00
parent 6739e47b15
commit 02bdcdd326
1 changed files with 12 additions and 14 deletions
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26
docs/LangRef.html
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@ -2512,10 +2512,9 @@ b: unreachable
<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 @@ b: unreachable
</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
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 @@ second_end:
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>