Alias Analysis Infrastructure in LLVM

From: Chris Lattner Date: Tue, 4 Mar 2003 19:37:49 +0000 (+0000) Subject: Initial checkin of AliasAnalysis doc X-Git-Url: http://plrg.eecs.uci.edu/git/?a=commitdiff_plain;h=9f6487563c0ea0e80b8c18a0e68fed4ade43f269;p=oota-llvm.git Initial checkin of AliasAnalysis doc git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@5696 91177308-0d34-0410-b5e6-96231b3b80d8 --- diff --git a/docs/AliasAnalysis.html b/docs/AliasAnalysis.html new file mode 100644 index 00000000000..1cbd5a17992 --- /dev/null +++ b/docs/AliasAnalysis.html @@ -0,0 +1,451 @@ + +Alias Analysis Infrastructure in LLVM + + + + + +

Alias Analysis Infrastructure in LLVM

+ +

Introduction + +
AliasAnalysis Overview +
+ +
Writing a new AliasAnalysis Implementation +
+ +
Using AliasAnalysis results +
+
Helpful alias analysis related tools +
+ + +
Written by Chris Lattner
+

+ + + + + +
+Introduction +

Must, May, or No

+ +The AliasAnalysis class is the +centerpiece of the LLVM Alias Analysis related infrastructure. This class is +the common interface between clients of alias analysis information and the +implementations providing it. In addition to simple alias analysis information, +this class exposes Mod/Ref information from those implementations which can +provide it, allowing for powerful analyses and transformations to work well +together.

+ +This document contains information neccesary to successfully implement this +interface, use it, and to test both sides. It also explains some of the finer +points about what exactly results mean. If you feel that something is unclear +or should be added, please let me know.

+ + + +

+AliasAnalysis Overview +

AliasAnalysis

AliasResult

ModRefResult

+ +The AliasAnalysis interface exposes information about memory, represented in +several different ways. In particular, memory objects are represented as a +starting address and size, and function calls are represented as the actual +call or invoke instructions that performs the call. The +AliasAnalysis interface also exposes some helper methods which allow you to get +mod/ref information for arbitrary instructions.

+ + +

+ +Representation of Pointers +

+ +Representing memory objects as a starting address and a size is critically +important for precise Alias Analyses. For example, consider this (silly) C +code:

+ +

+  int i;
+  char C[2];
+  char A[10]; 
+  /* ... */
+  for (i = 0; i != 10; ++i) {
+    C[0] = A[i];          /* One byte store */
+    C[1] = A[9-i];        /* One byte store */
+  }
+

basicaa

C[0]

C[1]

+ +

+  int i;
+  char C[2];
+  char A[10]; 
+  /* ... */
+  for (i = 0; i != 10; ++i) {
+    ((short*)C)[0] = A[i];  /* Two byte store! */
+    C[1] = A[9-i];          /* One byte store */
+  }
+

&C[0]

+ + + +

+ +Must, May, and No Alias Responses +

+ +The Must Alias response is trickier though. In LLVM, the Must Alias response +may only be returned if the two memory objects are guaranteed to always start at +exactly the same location. If two memory objects overlap, but do not start at +the same location, MayAlias must be returned.

+ + + +

+ +The getModRefInfo methods +

getModRefInfo

may

+ + + + +

+Writing a new AliasAnalysis Implementation +

no-aa

+ + + +

+ +Different Pass styles +

LLVM +pass

+ +

If you require interprocedural analysis, it should be a Pass. +
If you are a global analysis, subclass FunctionPass. +
If you are a local pass, subclass BasicBlockPass. +
If you don't need to look at the program at all, subclass + ImmutablePass. +

+ +In addition to the pass that you subclass, you should also inherit from the +AliasAnalysis interface of course, and use the +RegisterAnalysisGroup template to register as an implementation of +AliasAnalysis.

+ + + +

+ +Required initialization calls +

getAnalysisUsage

InitializeAliasAnalysis

getAnalysisUsage

AliasAnalysis::getAnalysisUsage

+ +

+    void getAnalysisUsage(AnalysisUsage &AU) const {
+      AliasAnalysis::getAnalysisUsage(AU);
+      // declare your dependencies here.
+    }
+

InitializeAliasAnalysis

run

Pass

runOnFunction

FunctionPass

runOnBasicBlock

BasicBlockPass

InitializeAliasAnalysis

ImmutablePass

Pass

+ +

+    bool run(Module &M) {
+      InitializeAliasAnalysis(this);
+      // Perform analysis here...
+      return false;
+    }
+

+ +Interfaces which may be specified +

AliasAnalysis

+ +The AliasAnalysis chaining behavior +

basicaa

no-aa

-basic-aa -ds-aa -andersens-aa -licm

getAnalysisUsage

+ + + +

+ +Efficiency Issues +

queries

+ + + +

+Using AliasAnalysis results +

+ + +

+ +Using the -load-vn Pass +

load-vn

load

load-vn

+ + + +

+ +Using the AliasSetTracker class +

sets

AliasSetTracker

+ +First you initialize the AliasSetTracker by use the "add" methods to +add information about various potentially aliasing instructions in the scope you +are interested in. Once all of the alias sets are completed, your pass should +simply iterate through the constructed alias sets, using the AliasSetTracker +begin()/end() methods.

+ +The AliasSets formed by the AliasSetTracker are guaranteed to +be disjoint, calculate mod/ref information for the set, and keep track of +whether or not all of the pointers in the set are Must aliases. The +AliasSetTracker also makes sure that sets are properly folded due to call +instructions, and can provide a list of pointers in each set.

+ +As an example user of this, the Loop +Invariant Code Motion pass uses AliasSetTrackers to build alias information +about each loop nest. If an AliasSet in a loop is not modified, then all load +instructions from that set may be hoisted out of the loop. If any alias sets +are stored and are must alias sets, then the stores may be sunk to +outside of the loop. Both of these transformations obviously only apply if the +pointer argument is loop-invariant.

+ + + +

+ +Using the AliasAnalysis interface directly +

let me know

+ + + +

+Helpful alias analysis related tools +

+ + +

+ +The -no-aa pass +

-no-aa

basicaa

+ + + +

+ +The -print-alias-sets pass +

-print-alias-sets

analyze

AliasSetTracker

+ + + +

+ +The -count-aa pass +

-count-aa

+ +

+  $ opt -basicaa -count-aa -ds-aa -count-aa -licm
+

-licm

-ds-aa

-basicaa

-ds-aa

+ + +

+ +The -aa-eval pass +

-aa-eval

+ + + +

+ + +

Chris Lattner

+ + +Last modified: Tue Mar 4 13:36:53 CST 2003 + +