improve some of the documentation around target data layout strings.
git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@141733 91177308-0d34-0410-b5e6-96231b3b80d8
diff --git a/docs/LangRef.html b/docs/LangRef.html
index 145c19f..8e54f3c 100644
--- a/docs/LangRef.html
+++ b/docs/LangRef.html
@@ -1406,6 +1406,22 @@
implemented in terms of 64 <2 x double>, for example.</li>
</ol>
+<p>The function of the data layout string may not be what you expect. Notably,
+ this is not a specification from the frontend of what alignment the code
+ generator should use.</p>
+
+<p>Instead, if specified, the target data layout is required to match what the
+ ultimate <em>code generator</em> expects. This string is used by the
+ mid-level optimizers to
+ improve code, and this only works if it matches what the ultimate code
+ generator uses. If you would like to generate IR that does not embed this
+ target-specific detail into the IR, then you don't have to specify the
+ string. This will disable some optimizations that require precise layout
+ information, but this also prevents those optimizations from introducing
+ target specificity into the IR.</p>
+
+
+
</div>
<!-- ======================================================================= -->
diff --git a/include/llvm/Target/TargetData.h b/include/llvm/Target/TargetData.h
index e34094d..26fd187 100644
--- a/include/llvm/Target/TargetData.h
+++ b/include/llvm/Target/TargetData.h
@@ -44,6 +44,7 @@
AGGREGATE_ALIGN = 'a', ///< Aggregate alignment
STACK_ALIGN = 's' ///< Stack objects alignment
};
+
/// Target alignment element.
///
/// Stores the alignment data associated with a given alignment type (pointer,
@@ -64,6 +65,12 @@
bool operator==(const TargetAlignElem &rhs) const;
};
+/// TargetData - This class holds a parsed version of the target data layout
+/// string in a module and provides methods for querying it. The target data
+/// layout string is specified *by the target* - a frontend generating LLVM IR
+/// is required to generate the right target data for the target being codegen'd
+/// to. If some measure of portability is desired, an empty string may be
+/// specified in the module.
class TargetData : public ImmutablePass {
private:
bool LittleEndian; ///< Defaults to false
