[llvm-commits] [llvm] r125412 - /llvm/trunk/docs/GetElementPtr.html
Chris Lattner
sabre at nondot.org
Fri Feb 11 13:50:52 PST 2011
Author: lattner
Date: Fri Feb 11 15:50:52 2011
New Revision: 125412
URL: http://llvm.org/viewvc/llvm-project?rev=125412&view=rev
Log:
attempt to capture recent discussion about overflow and inbounds geps.
Modified:
llvm/trunk/docs/GetElementPtr.html
Modified: llvm/trunk/docs/GetElementPtr.html
URL: http://llvm.org/viewvc/llvm-project/llvm/trunk/docs/GetElementPtr.html?rev=125412&r1=125411&r2=125412&view=diff
==============================================================================
--- llvm/trunk/docs/GetElementPtr.html (original)
+++ llvm/trunk/docs/GetElementPtr.html Fri Feb 11 15:50:52 2011
@@ -598,13 +598,27 @@
<a name="overflow"><b>What happens if a GEP computation overflows?</b></a>
</div>
<div class="doc_text">
- <p>If the GEP has the <tt>inbounds</tt> keyword, the result value is
- undefined.</p>
-
- <p>Otherwise, the result value is the result from evaluating the implied
- two's complement integer computation. However, since there's no
- guarantee of where an object will be allocated in the address space,
- such values have limited meaning.</p>
+ <p>If the GEP lacks the <tt>inbounds</tt> keyword, the value is the result
+ from evaluating the implied two's complement integer computation. However,
+ since there's no guarantee of where an object will be allocated in the
+ address space, such values have limited meaning.</p>
+
+ <p>If the GEP has the <tt>inbounds</tt> keyword, the result value is
+ undefined (a "<a href="LangRef.html#trapvalues">trap value</a>") if the GEP
+ overflows (i.e. wraps around the end of the address space).</p>
+
+ <p>As such, there are some ramifications of this for inbounds GEPs: scales
+ implied by array/vector/pointer indices are always known to be "nsw" since
+ they are signed values that are scaled by the element size. These values
+ are also allowed to be negative (e.g. "gep i32 *%P, i32 -1") but the
+ pointer itself is logically treated as an unsigned value. This means that
+ GEPs have an asymmetric relation between the pointer base (which is treated
+ as unsigned) and the offset applied to it (which is treated as signed). The
+ result of the additions within the offset calculation cannot have signed
+ overflow, but when applied to the base pointer, there can be signed
+ overflow.
+ </p>
+
</div>
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