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improve some of the documentation around target data layout strings.

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git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@141733 91177308-0d34-0410-b5e6-96231b3b80d8
This commit is contained in:
Chris Lattner 2011-10-11 23:01:39 +00:00
parent 6618a241f7
commit 6509f50c69
2 changed files with 23 additions and 0 deletions
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16
docs/LangRef.html
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@ -1406,6 +1406,22 @@ target datalayout = "<i>layout specification</i>"
implemented in terms of 64 &lt;2 x double&gt;, 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>
<!-- ======================================================================= -->

7
include/llvm/Target/TargetData.h
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@ -44,6 +44,7 @@ enum AlignTypeEnum {
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 @@ struct TargetAlignElem {
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