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Following discussion on llvm-dev ("proposed new rule for getelementptr"),
add a new "Pointer Aliasing Rules" section. git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@77216 91177308-0d34-0410-b5e6-96231b3b80d8
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@ -48,6 +48,7 @@
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<li><a href="#gc">Garbage Collector Names</a></li>
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<li><a href="#moduleasm">Module-Level Inline Assembly</a></li>
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<li><a href="#datalayout">Data Layout</a></li>
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<li><a href="#pointeraliasing">Pointer Aliasing Rules</a></li>
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</ol>
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</li>
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<li><a href="#typesystem">Type System</a>
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@ -1243,6 +1244,54 @@ target datalayout = "<i>layout specification</i>"
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</div>
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<!-- ======================================================================= -->
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<div class="doc_subsection">
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<a name="pointeraliasing">Pointer Aliasing Rules</a>
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</div>
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<div class="doc_text">
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<p>Any memory access must be done though a pointer value associated
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with with address range of the memory access, otherwise the behavior
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is undefined. Pointer values are associated with address ranges
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according to the following rules:</p>
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<ul>
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<li>A pointer value formed from a <tt>getelementptr</tt> instruction is
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associated with the addresses associated with the first operand of
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the <tt>getelementptr</tt>.</li>
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<li>An addresses of a global variable is associated with the address
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range of the variable's storage.</li>
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<li>The result value of an allocation instruction is associated with
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the address range of the allocated storage.</li>
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<li>A null pointer in the default address-space is associated with
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no addresses.</li>
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<li>A pointer value formed by an <tt>inttoptr</tt> is associated with
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all address ranges of all pointer values that contribute (directly
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or indirectly) to the computation of the pointer's value.</li>
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<li>The result value of a <tt>bitcast</tt> is associated with all
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addresses associated with the operand of the <tt>bitcast</tt>.</li>
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<li>An integer constant other than zero or a pointer value returned
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from a function not defined within LLVM may be associated with address
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ranges allocated through mechanisms other than those provided by
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LLVM. Such ranges shall not overlap with any ranges of address
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allocated by mechanisms provided by LLVM.</li>
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</ul>
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<p>LLVM IR does not associate types with memory. The result type of a
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<tt>load</tt> merely indicates the size and alignment of the memory from
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which to load, as well as the interpretation of the value. The first
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operand of a <tt>store</tt> similarly only indicates the size and
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alignment of the store.</p>
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<p>Consequently, type-based alias analysis, aka TBAA, aka
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<tt>-fstrict-aliasing</tt>, is not applicable to general unadorned
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LLVM IR. <a href="#metadata">Metadata</a> may be used to encode
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additional information which specialized optimization passes may use
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to implement type-based alias analysis.</p>
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</div>
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<!-- *********************************************************************** -->
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<div class="doc_section"> <a name="typesystem">Type System</a> </div>
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<!-- *********************************************************************** -->
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