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Diffstat (limited to 'docs/GetElementPtr.html')
| -rw-r--r-- | docs/GetElementPtr.html | 30 |
1 files changed, 22 insertions, 8 deletions
diff --git a/docs/GetElementPtr.html b/docs/GetElementPtr.html index d840c97..5410137 100644 --- a/docs/GetElementPtr.html +++ b/docs/GetElementPtr.html @@ -598,13 +598,27 @@ idx3 = (char*) &MyVar + 8 <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> @@ -719,7 +733,7 @@ idx3 = (char*) &MyVar + 8 <a href="http://validator.w3.org/check/referer"><img src="http://www.w3.org/Icons/valid-html401-blue" alt="Valid HTML 4.01"></a> <a href="http://llvm.org">The LLVM Compiler Infrastructure</a><br/> - Last modified: $Date: 2010-08-28 06:09:24 +0200 (Sat, 28 Aug 2010) $ + Last modified: $Date: 2011-02-11 22:50:52 +0100 (Fri, 11 Feb 2011) $ </address> </body> </html> |
