[llvm-commits] [llvm] r102140 - /llvm/trunk/docs/LangRef.html

[Chris Lattner]

Looks great Dan, thanks for clarifying this!

On Apr 22, 2010, at 4:14 PM, Dan Gohman wrote:

> Author: djg
> Date: Thu Apr 22 18:14:21 2010
> New Revision: 102140
> 
> URL: http://llvm.org/viewvc/llvm-project?rev=102140&view=rev
> Log:
> Add an initial description of a new concept: trap values, and change
> the definition of the nsw and nuw flags to make use of it.
> 
> nsw was introduced to help optimizers answer yes to the following:
> 
>  // Can we change i from i32 to i64 to eliminate the cast inside the loop?
>  for (int i = 0; i < n; ++i) A[i] *= 0.1;
> 
>  // Can we assume that this loop will eventually terminate?
>  for (int i = 0; i <= n; ++i) A[i] *= 0.1;
> 
> In its current form, it isn't truly sufficient for either.
> 
> In the first case, if the increment overflows, it'll still have some
> valid i32 value; sign-extending it will produce a value which is 33
> homogeneous sign bits trailed by 31 independent undef bits. If i is
> promoted to i64, it won't have those same values when it reaches that
> point. (The compiler could recover here by reasoning about how i is
> used by the load, but that's a lot more complicated and isn't always
> possible.)
> 
> In the second case, there is no value for i which will be greater than
> n, so having the increment return undef on overflow doesn't help.
> 
> Trap values are a formalization of some existing concepts that we have
> about LLVM IR, and give the optimizers a better basis for answering yes
> to both questions above.
> 
> 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=102140&r1=102139&r2=102140&view=diff
> ==============================================================================
> --- llvm/trunk/docs/LangRef.html (original)
> +++ llvm/trunk/docs/LangRef.html Thu Apr 22 18:14:21 2010
> @@ -89,6 +89,7 @@
>       <li><a href="#complexconstants">Complex Constants</a></li>
>       <li><a href="#globalconstants">Global Variable and Function Addresses</a></li>
>       <li><a href="#undefvalues">Undefined Values</a></li>
> +      <li><a href="#trapvalues">Trap Values</a></li>
>       <li><a href="#blockaddress">Addresses of Basic Blocks</a></li>
>       <li><a href="#constantexprs">Constant Expressions</a></li>
>     </ol>
> @@ -2303,6 +2304,34 @@
> </div>
> 
> <!-- ======================================================================= -->
> +<div class="doc_subsection"><a name="trapvalues">Trap Values</a></div>
> +<div class="doc_text">
> +
> +<p>Trap 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>
> +
> +<p>Any non-void instruction or constant expression other than non-intrinsic
> +   calls or invokes with a trap operand has trap as its result value.
> +   Any instruction with a trap operand which may have side effects emits
> +   those side effects as if it had an undef operand instead.</p>
> +
> +<p>For example, an <a href="#i_and"><tt>and</tt></a> of a trap value with
> +   zero still has a trap value result. Using that value as an index in a
> +   <a href="#i_getelementptr"><tt>getelementptr</tt></a> yields a trap
> +   result. Using that result as the address of a
> +   <a href="#i_store"><tt>store</tt></a> produces undefined behavior.</p>
> +
> +<p>There is currently no way of representing a trap constant in the IR; they
> +   only exist when produced by certain instructions, such as an
> +   <a href="#i_add"><tt>add</tt></a> with the <tt>nsw</tt> flag
> +   set, when overflow occurs.</p>
> +
> +</div>
> +
> +<!-- ======================================================================= -->
> <div class="doc_subsection"><a name="blockaddress">Addresses of Basic
>     Blocks</a></div>
> <div class="doc_text">
> @@ -3104,7 +3133,8 @@
> <p><tt>nuw</tt> and <tt>nsw</tt> stand for "No Unsigned Wrap"
>    and "No Signed Wrap", respectively. If the <tt>nuw</tt> and/or
>    <tt>nsw</tt> keywords are present, the result value of the <tt>add</tt>
> -   is undefined if unsigned and/or signed overflow, respectively, occurs.</p>
> +   is a <a href="#trapvalues">trap value</a> if unsigned and/or signed overflow,
> +   respectively, occurs.</p>
> 
> <h5>Example:</h5>
> <pre>
> @@ -3184,7 +3214,8 @@
> <p><tt>nuw</tt> and <tt>nsw</tt> stand for "No Unsigned Wrap"
>    and "No Signed Wrap", respectively. If the <tt>nuw</tt> and/or
>    <tt>nsw</tt> keywords are present, the result value of the <tt>sub</tt>
> -   is undefined if unsigned and/or signed overflow, respectively, occurs.</p>
> +   is a <a href="#trapvalues">trap value</a> if unsigned and/or signed overflow,
> +   respectively, occurs.</p>
> 
> <h5>Example:</h5>
> <pre>
> @@ -3270,7 +3301,8 @@
> <p><tt>nuw</tt> and <tt>nsw</tt> stand for "No Unsigned Wrap"
>    and "No Signed Wrap", respectively. If the <tt>nuw</tt> and/or
>    <tt>nsw</tt> keywords are present, the result value of the <tt>mul</tt>
> -   is undefined if unsigned and/or signed overflow, respectively, occurs.</p>
> +   is a <a href="#trapvalues">trap value</a> if unsigned and/or signed overflow,
> +   respectively, occurs.</p>
> 
> <h5>Example:</h5>
> <pre>
> 
> 
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