DOC: Add first version of updated documentation.
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Travis E. Oliphant
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19 changed files with 474 additions and 595 deletions
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@ -1,12 +1,16 @@
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+------------------------+
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| layout: page |
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+------------------------+
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| title: LLVM Concepts |
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+------------------------+
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********************
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LLVM Concepts
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********************
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This section explains a few concepts related to LLVM, not specific to
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llvmpy.
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.. toctree::
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:hidden:
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Intermediate Representation
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===========================
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@ -36,7 +40,6 @@ instructions related to variable argument handling, exception handling,
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and garbage collection. These allow high-level languages to be
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represented cleanly in the IR.
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--------------
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SSA Form and PHI Nodes
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======================
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@ -51,15 +54,24 @@ when a variable can be assigned a different value based on the path of
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control flow. For example, the value of *b* at the end of execution of
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the snippet below:
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{% highlight c %} a = 1; if (v < 10) a = 2; b = a; {% endhighlight %}
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.. code-block:: c
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a = 1;
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if (v < 10)
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a = 2;
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b = a;
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cannot be determined statically. The value of '2' cannot be assigned to
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the 'original' *a*, since *a* can be assigned to only once. There are
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two *a* 's in there, and the last assignment has to choose between which
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version to pick. This is accomplished by adding a PHI node:
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{% highlight c %} a1 = 1; if (v < 10) a2 = 2; b = PHI(a1, a2); {%
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endhighlight %}
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.. code-block:: c
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a1 = 1;
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if (v < 10)
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a2 = 2;
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b = PHI(a1, a2);
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The PHI node selects *a1* or *a2*, depending on where the control
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reached the PHI node. The argument *a1* of the PHI node is associated
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@ -68,7 +80,6 @@ with the block *"a1 = 1;"* and *a2* with the block *"a2 = 2;"*.
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PHI nodes have to be explicitly created in the LLVM IR. Accordingly the
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LLVM instruction set has an instruction called *phi*.
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--------------
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LLVM Assembly Language
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======================
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@ -90,30 +101,48 @@ language code.
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Just to get a feel of the LLVM assembly language, here's a function in
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C, and the corresponding LLVM assembly (as generated by the demo page):
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{% highlight c %} /\* compute sum of 1..n \*/ unsigned sum(unsigned n) {
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if (n == 0) return 0; else return n + sum(n-1); } {% endhighlight %}
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.. code-block:: c
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/* compute sum of 1..n */
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unsigned sum(unsigned n) {
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if (n == 0)
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return 0;
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else
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return n + sum(n-1);
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}
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The corresponding LLVM assembly:
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{% highlight llvm %} ; ModuleID = '/tmp/webcompile/\_7149\_0.bc' target
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datalayout =
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"e-p:64:64:64-i1:8:8-i8:8:8-i16:16:16-i32:32:32-i64:64:64-f32:32:32-f64:64:64-v64:64:64-v128:128:128-a0:0:64-s0:64:64-f80:128:128-n8:16:32:64"
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target triple = "x86\_64-linux-gnu"
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.. code-block:: llvm
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define i32 @sum(i32 %n) nounwind readnone { entry: %0 = icmp eq i32 %n,
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0 ; [#uses=1] br i1 %0, label %bb2, label %bb1
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; ModuleID = '/tmp/webcompile/_7149_0.bc'
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target datalayout = "e-p:64:64:64-i1:8:8-i8:8:8-i16:16:16-i32:32:32-i64:64:64-f32:32:32-f64:64:64-v64:64:64-v128:128:128-a0:0:64-s0:64:64-f80:128:128-n8:16:32:64"
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target triple = "x86_64-linux-gnu"
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bb1: ; preds = %entry %1 = add i32 %n, -1 ; [#uses=2] %2 = icmp eq i32
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%1, 0 ; [#uses=1] br i1 %2, label %sum.exit, label %bb1.i
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define i32 @sum(i32 %n) nounwind readnone {
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entry:
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%0 = icmp eq i32 %n, 0 ; [#uses=1]
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br i1 %0, label %bb2, label %bb1
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bb1.i: ; preds = %bb1 %3 = add i32 %n, -2 ; [#uses=1] %4 = tail call i32
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@sum(i32 %3) nounwind ; [#uses=1] %5 = add i32 %4, %1 ; [#uses=1] br
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label %sum.exit
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bb1: ; preds = %entry
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%1 = add i32 %n, -1 ; [#uses=2]
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%2 = icmp eq i32 %1, 0 ; [#uses=1]
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br i1 %2, label %sum.exit, label %bb1.i
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sum.exit: ; preds = %bb1.i, %bb1 %6 = phi i32 [ %5, %bb1.i ], [ 0, %bb1
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] ; [#uses=1] %7 = add i32 %6, %n ; [#uses=1] ret i32 %7
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bb1.i: ; preds = %bb1
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%3 = add i32 %n, -2 ; [#uses=1]
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%4 = tail call i32 @sum(i32 %3) nounwind ; [#uses=1]
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%5 = add i32 %4, %1 ; [#uses=1]
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br label %sum.exit
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bb2: ; preds = %entry ret i32 0 } {% endhighlight %}
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sum.exit: ; preds = %bb1.i, %bb1
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%6 = phi i32 [ %5, %bb1.i ], [ 0, %bb1 ] ; [#uses=1]
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%7 = add i32 %6, %n ; [#uses=1]
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ret i32 %7
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bb2: ; preds = %entry
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ret i32 0
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}
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Note the usage of SSA form. The long string called ``target datalayout``
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is a specification of the platform ABI (like endianness, sizes of types,
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@ -122,7 +151,6 @@ alignment etc.).
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The `LLVM Language Reference <http://www.llvm.org/docs/LangRef.html>`_
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defines the LLVM assembly language including the entire instruction set.
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--------------
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Modules
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=======
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@ -139,7 +167,6 @@ contained within modules. Modules may be combined (linked) together to
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give a bigger resultant module. During this process LLVM attempts to
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reconcile the references between the combined modules.
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--------------
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Optimization and Passes
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=======================
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@ -184,7 +211,6 @@ LLVM defines two kinds of pass managers:
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`PassManager <http://llvm.org/docs/doxygen/html/classllvm_1_1PassManager.html>`_
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manages module passes for optimizing the entire module.
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--------------
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Bitcode
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=======
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@ -195,7 +221,6 @@ compiler <http://llvm.org/docs/LangRef.html#introduction>`_. See `LLVM
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documentation <http://llvm.org/docs/BitCodeFormat.html>`_ for detail
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about the bitcode format.
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--------------
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Execution Engine, JIT and Interpreter
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=====================================
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@ -209,6 +234,3 @@ multiple modules.
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Inter-module reference is not possible. That is module ``A`` cannot
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call a function in module ``B``, directly.
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--------------
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**Next** -- `llvmpy Package <./llvm-py_package.html>`_
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