6502/llvm-6502
1
0
Fork 0
mirror of https://github.com/c64scene-ar/llvm-6502.git synced 2024-08-21 18:29:46 +00:00
Code Issues Projects Releases Wiki Activity

Add the blurb about the new exception handling.

Browse Source 
git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@143042 91177308-0d34-0410-b5e6-96231b3b80d8
This commit is contained in:
Bill Wendling 2011-10-26 18:33:01 +00:00
parent 65d1f415c7
commit bc5f6ddfba
1 changed files with 108 additions and 3 deletions
Show Stats Download Patch File Download Diff File Expand all files Collapse all files

111
docs/ReleaseNotes.html
View File

@ -689,12 +689,117 @@ in this section.
<p>LLVM IR has several new features for better support of new targets and that
expose new optimization opportunities:</p>
<p>One of the biggest changes is that 3.0 has a new exception handling
system. The old system used LLVM intrinsics to convey the exception handling
information to the code generator. It worked in most cases, but not
all. Inlining was especially difficult to get right. Also, the intrinsics
could be moved away from the <code>invoke</code> instruction, making it hard
to recover that information.</p>
<p>The new EH system makes exception handling a first-class member of the IR. It
adds two new instructions:</p>
<ul>
<!--
<li></li>
-->
<li><a href="LangRef.html#i_landingpad"><code>landingpad</code></a> &mdash;
this instruction defines a landing pad basic block. It contains all of the
information that's needed by the code generator. It's also required to be
the first non-PHI instruction in the landing pad. In addition, a landing
pad may be jumped to only by the unwind edge of an <code>invoke</code>
instruction.</li>
<li><a href="LangRef.html#i_resume"><code>resume</code></a> &mdash; this
instruction causes the current exception to resume traveling up the
stack. It replaces the <code>@llvm.eh.resume</code> intrinsic.</li>
</ul>
<p>Converting from the old EH API to the new EH API is rather simple, because a
lot of complexity has been removed. The two intrinsics,
<code>@llvm.eh.exception</code> and <code>@llvm.eh.selector</code> have been
superceded by the <code>landingpad</code> instruction. Instead of generating
a call to <code>@llvm.eh.exception</code> and <code>@llvm.eh.selector</code>:
<div class="doc_code">
<pre>
Function *ExcIntr = Intrinsic::getDeclaration(TheModule,
Intrinsic::eh_exception);
Function *SlctrIntr = Intrinsic::getDeclaration(TheModule,
Intrinsic::eh_selector);
// The exception pointer.
Value *ExnPtr = Builder.CreateCall(ExcIntr, "exc_ptr");
std::vector&lt;Value*&gt; Args;
Args.push_back(ExnPtr);
Args.push_back(Builder.CreateBitCast(Personality,
Type::getInt8PtrTy(Context)));
<i>// Add selector clauses to Args.</i>
// The selector call.
Builder.CreateCall(SlctrIntr, Args, "exc_sel");
</pre>
</div>
<p>You should instead generate a <code>landingpad</code> instruction, that
returns an exception object and selector value:</p>
<div class="doc_code">
<pre>
LandingPadInst *LPadInst =
Builder.CreateLandingPad(StructType::get(Int8PtrTy, Int32Ty, NULL),
Personality, 0);
Value *LPadExn = Builder.CreateExtractValue(LPadInst, 0);
Builder.CreateStore(LPadExn, getExceptionSlot());
Value *LPadSel = Builder.CreateExtractValue(LPadInst, 1);
Builder.CreateStore(LPadSel, getEHSelectorSlot());
</pre>
</div>
<p>It's now trivial to add the individual clauses to the <code>landingpad</code>
instruction.</p>
<div class="doc_code">
<pre>
<i><b>// Adding a catch clause</b></i>
Constant *TypeInfo = getTypeInfo();
LPadInst-&gt;addClause(TypeInfo);
<i><b>// Adding a C++ catch-all</b></i>
LPadInst-&gt;addClause(Constant::getNullValue(Builder.getInt8PtrTy()));
<i><b>// Adding a cleanup</b></i>
LPadInst-&gt;setCleanup(true);
<i><b>// Adding a filter clause</b></i>
std::vector&lt;Constant*&gt; TypeInfos;
Constant *TypeInfo = getFilterTypeInfo();
TypeInfos.push_back(Builder.CreateBitCast(TypeInfo, Builder.getInt8PtrTy()));
ArrayType *FilterTy = ArrayType::get(Int8PtrTy, TypeInfos.size());
LPadInst-&gt;addClause(ConstantArray::get(FilterTy, TypeInfos));
</pre>
</div>
<p>Converting from using the <code>@llvm.eh.resume</code> intrinsic to
the <code>resume</code> instruction is trivial. It takes the exception
pointer and exception selector values returned by
the <code>landingpad</code> instruction:</p>
<div class="doc_code">
<pre>
Type *UnwindDataTy = StructType::get(Builder.getInt8PtrTy(),
Builder.getInt32Ty(), NULL);
Value *UnwindData = UndefValue::get(UnwindDataTy);
Value *ExcPtr = Builder.CreateLoad(getExceptionObjSlot());
Value *ExcSel = Builder.CreateLoad(getExceptionSelSlot());
UnwindData = Builder.CreateInsertValue(UnwindData, ExcPtr, 0, "exc_ptr");
UnwindData = Builder.CreateInsertValue(UnwindData, ExcSel, 1, "exc_sel");
Builder.CreateResume(UnwindData);
</pre>
</div>
</div>
<!--=========================================================================-->