llvm dev - Nov 2011 - [LLVMdev] Adding a custom GC safe point creation phase

Hi Chris, Gordon,

Here's a patch to allow a GCStrategy to customize the places where it wants
to insert safe points. I'm not sure who maintains the GC code today in LLVM
(I'd be happy to take ownership, if needed).

The patch just adds up a custom safepoints flag, similar to the way the
GCStrategy can customize intrinsics lowering, or roots initialization. It
works pretty well, as I've tested it on VMKit. So the patch in itself
should not be controversial.

Chris, I'd like to discuss with you on how we can improve the current GC
framework to take advantage of metadata. The reason I add this custom safe
point creation method is because it's difficult for a front-end to pass
custom information to the GC framework. And I think metadata could help us
in this situation. Ideally, I'd like to get to a point where we can write:

15 = load %MyPointer* %2 !safepoint

And the GCStrategy would find this metadata information in the
MachineInstruction (because the safe points are created based on
MachineInstruction). This solution looks very much like the !nontemporal
metadata.

Unfortunately, it looks like metadata are not passed down to
MachineInstruction. How could we achieve that? Should we write custom code
in the Instruction -> MachineInstruction transformation to pass down that
!safepoint metadata?

Thanks for any input!
Nicolas


Index: include/llvm/CodeGen/GCStrategy.h
==================================================================---
include/llvm/CodeGen/GCStrategy.h (revision 141409)
+++ include/llvm/CodeGen/GCStrategy.h (working copy)
@@ -37,6 +37,7 @@
 #define LLVM_CODEGEN_GCSTRATEGY_H

 #include "llvm/CodeGen/GCMetadata.h"
+#include "llvm/CodeGen/MachineFunction.h"
 #include "llvm/Support/Registry.h"
 #include <string>

@@ -68,6 +69,8 @@
     bool CustomReadBarriers;   //< Default is to insert loads.
     bool CustomWriteBarriers;  //< Default is to insert stores.
     bool CustomRoots;          //< Default is to pass through to backend.
+    bool CustomSafePoints;     //< Default is to use NeededSafePoints
+                               //  to find safe points.
     bool InitRoots;            //< If set, roots are nulled during
lowering.
     bool UsesMetadata;         //< If set, backend must emit metadata
tables.

@@ -87,7 +90,9 @@

     /// needsSafePoitns - True if safe points of any kind are required. By
     //                    default, none are recorded.
-    bool needsSafePoints() const { return NeededSafePoints != 0; }
+    bool needsSafePoints() const {
+      return CustomSafePoints || NeededSafePoints != 0;
+    }

     /// needsSafePoint(Kind) - True if the given kind of safe point is
     //                          required. By default, none are recorded.
@@ -109,6 +114,11 @@
     ///               can generate a stack map. If true, then
     //                performCustomLowering must delete them.
     bool customRoots() const { return CustomRoots; }
+
+    /// customSafePoints - By default, the GC analysis will find safe
+    ///                    points according to NeededSafePoints. If true,
+    ///                    then findCustomSafePoints must create them.
+    bool customSafePoints() const { return CustomSafePoints; }

     /// initializeRoots - If set, gcroot intrinsics should initialize their
     //                    allocas to null before the first use. This is
@@ -135,6 +145,7 @@
     /// which the LLVM IR can be modified.
     virtual bool initializeCustomLowering(Module &F);
     virtual bool performCustomLowering(Function &F);
+    virtual bool findCustomSafePoints(GCFunctionInfo& FI,
MachineFunction&
MF);
   };

 }
Index: lib/CodeGen/GCStrategy.cpp
==================================================================---
lib/CodeGen/GCStrategy.cpp (revision 141409)
+++ lib/CodeGen/GCStrategy.cpp (working copy)
@@ -97,6 +97,7 @@
   CustomReadBarriers(false),
   CustomWriteBarriers(false),
   CustomRoots(false),
+  CustomSafePoints(false),
   InitRoots(true),
   UsesMetadata(false)
 {}
@@ -116,6 +117,14 @@
   return 0;
 }

+
+bool GCStrategy::findCustomSafePoints(GCFunctionInfo& FI, MachineFunction
&F) {
+  dbgs() << "gc " << getName() << " must
override findCustomSafePoints.\n";
+  llvm_unreachable(0);
+  return 0;
+}
+
+
 GCFunctionInfo *GCStrategy::insertFunctionInfo(const Function &F) {
   GCFunctionInfo *FI = new GCFunctionInfo(F, *this);
   Functions.push_back(FI);
@@ -370,6 +379,7 @@
     MCSymbol* Label = InsertLabel(*CI->getParent(), RAI,
CI->getDebugLoc());
     FI->addSafePoint(GC::PostCall, Label, CI->getDebugLoc());
   }
+
 }

 void MachineCodeAnalysis::FindSafePoints(MachineFunction &MF) {
@@ -405,9 +415,13 @@

   // Find the size of the stack frame.
   FI->setFrameSize(MF.getFrameInfo()->getStackSize());
-
+
   // Find all safe points.
-  FindSafePoints(MF);
+  if (FI->getStrategy().customSafePoints()) {
+    FI->getStrategy().findCustomSafePoints(*FI, MF);
+  } else {
+    FindSafePoints(MF);
+  }

   // Find the stack offsets for all roots.
   FindStackOffsets(MF);
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On 2011-10-31, at 17:21, Nicolas Geoffray wrote:
> Here's a patch to allow a GCStrategy to customize the places where it
wants to insert safe points. I'm not sure who maintains the GC code today in
LLVM (I'd be happy to take ownership, if needed).
> 
> The patch just adds up a custom safepoints flag, similar to the way the
GCStrategy can customize intrinsics lowering, or roots initialization. It works
pretty well, as I've tested it on VMKit. So the patch in itself should not
be controversial.
Seems a perfectly reasonable patch on its own merits.

I would be curious what your findCustomSafePoints implementation looks like.
Perhaps it's possible to extract some of it into the framework.
> Chris, I'd like to discuss with you on how we can improve the current
GC framework to take advantage of metadata. The reason I add this custom safe
point creation method is because it's difficult for a front-end to pass
custom information to the GC framework. And I think metadata could help us in
this situation. Ideally, I'd like to get to a point where we can write:
> 
> 15 = load %MyPointer* %2 !safepoint
> 
> And the GCStrategy would find this metadata information in the
MachineInstruction (because the safe points are created based on
MachineInstruction). This solution looks very much like the !nontemporal
metadata.
> 
> Unfortunately, it looks like metadata are not passed down to
MachineInstruction. How could we achieve that? Should we write custom code in
the Instruction -> MachineInstruction transformation to pass down that
!safepoint metadata?
Could such a program survive IR optimizations intact?

— Gordon

Thanks for the review Gordon.

On Tue, Nov 1, 2011 at 2:21 AM, Gordon Henriksen <gordonhenriksen at
mac.com>wrote:
> On 2011-10-31, at 17:21, Nicolas Geoffray wrote:
>
> > Here's a patch to allow a GCStrategy to customize the places where
it
> wants to insert safe points. I'm not sure who maintains the GC code
today
> in LLVM (I'd be happy to take ownership, if needed).
> >
> > The patch just adds up a custom safepoints flag, similar to the way
the
> GCStrategy can customize intrinsics lowering, or roots initialization. It
> works pretty well, as I've tested it on VMKit. So the patch in itself
> should not be controversial.
>
> Seems a perfectly reasonable patch on its own merits.
>
> I would be curious what your findCustomSafePoints implementation looks
> like. Perhaps it's possible to extract some of it into the framework.
>
The code for my findCustomSafePoints is like this:

void MyGC::findCustomSafePoints(GCFunctionInfo& FI, MachineFunction &MF)
{
  for (MachineFunction::iterator BBI = MF.begin(),
                                 BBE = MF.end(); BBI != BBE; ++BBI) {
    for (MachineBasicBlock::iterator MI = BBI->begin(),
                                     ME = BBI->end(); MI != ME; ++MI) {
      if (MI->getDesc().isCall()) {
               /// Standard code need for adding a post call safe point;
      } else if (MI->getDebucLoc().getCol() == 1) {
              /// Standard code need for adding a post call safe point;
      }
  }
}

So it really looks like what we already have, except this special trick
about checking the debug location. Piggy backing on the debug location is
easy and works for me because:
1) Debug locations are passed from LLVM Instructions to MachineInstruction
2) I don't have column numbers in my front-end.

The instructions that get this special marker on the debug location are
instructions that may trigger a segmentation fault. Because I enter a
signal handler, that may invoke a collection, I need to know the roots at
the faulting instruction's address.

> > Chris, I'd like to discuss with you on how we can improve the
current GC
> framework to take advantage of metadata. The reason I add this custom safe
> point creation method is because it's difficult for a front-end to pass
> custom information to the GC framework. And I think metadata could help us
> in this situation. Ideally, I'd like to get to a point where we can
write:
> >
> > 15 = load %MyPointer* %2 !safepoint
> >
> > And the GCStrategy would find this metadata information in the
> MachineInstruction (because the safe points are created based on
> MachineInstruction). This solution looks very much like the !nontemporal
> metadata.
> >
> > Unfortunately, it looks like metadata are not passed down to
> MachineInstruction. How could we achieve that? Should we write custom code
> in the Instruction -> MachineInstruction transformation to pass down
that
> !safepoint metadata?
>
> Could such a program survive IR optimizations intact?
>
Well, we must be sure that any instruction that is a safepoint does get
some metadata associated with it. If such instruction does not get this
metadata, then then program would not survive. So the 'safepoint'
metadata
*must* be preserved across optimizations if the instruction gets emitted.

Nicolas

> — Gordon
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