[LLVMdev] Supporting pre-allocated registers in LLVM

[Bill Wendling]

On Oct 9, 2007, at 1:40 AM, nkavv at physics.auth.gr wrote:

> i would like to have a look to your register allocator. Is it on  
> LLVM SVN? Or is
> it something standalone?
>
> BTW, my time is limited right now (writing up my Ph.D. as well as a  
> processor
> soft core) but i will dedicate some time (after all i do this for  
> polishing my
> Machine-SUIF passes, and there is no help there). LLVM has a  
> community, really
> working on advancing the compiler.
>
Hi Nikolaos,

The code isn't in SVN right now. Here is the code (put BitMatrix.h in  
include/llvm/ADT and RegAllocGraphColoring.cpp in lib/CodeGen).  
There's a few problems with it. For one, when calculating the  
"forbidden registers", it's getting confused because LLVM marks  
registers that are killed by a call as having a live interval. These  
interfere with the live intervals that the RA is handling, resulting  
in a large number of intervals that are uncolorable. I think I know a  
way around this, but haven't tried it yet.

After that's finished, the next thing will be to have a better  
heuristic for selecting a physical register for a live interval. The  
one it uses now is *really* dumb.

Take a look and let me know what you think or if you have any  
improvements!

-bw

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Index: include/llvm/CodeGen/Passes.h
===================================================================
--- include/llvm/CodeGen/Passes.h       (revision 42822)
+++ include/llvm/CodeGen/Passes.h       (working copy)
@@ -71,6 +71,9 @@
    ///
    FunctionPass *createLocalRegisterAllocator();
+  /// Graph Coloring Register Allocator Pass
+  FunctionPass *createGraphColoringRegisterAllocator();
+
    /// BigBlockRegisterAllocation Pass - The BigBlock register  
allocator
    /// munches single basic blocks at a time, like the local register
    /// allocator.  While the BigBlock allocator is a little slower,  
and uses
Index: include/llvm/CodeGen/LinkAllCodegenComponents.h
===================================================================
--- include/llvm/CodeGen/LinkAllCodegenComponents.h     (revision 42822)
+++ include/llvm/CodeGen/LinkAllCodegenComponents.h     (working copy)
@@ -34,6 +34,8 @@
        (void) llvm::createLinearScanRegisterAllocator();
        (void) llvm::createSimpleRegisterCoalescer();
+
+      (void) llvm::createGraphColoringRegisterAllocator();

        (void) llvm::createBURRListDAGScheduler(NULL, NULL, NULL);
        (void) llvm::createTDRRListDAGScheduler(NULL, NULL, NULL);