Fix bug causing loop unrolling to be disabled in some cases for loops with '#pragma clang loop unroll_count(N)"

[meheff]

Summary

After unrolling a loop with llvm.loop.unroll.count metadata (unroll factor hint) the loop unroller replaces the llvm.loop.unroll.count metadata with llvm.loop.unroll.disable metadata to prevent any subsequent unrolling passes from unrolling more than the hint indicates. However, if the unroll count metadata is shared between loops then unrolling for these other loops will also be disabled. This small patch fixes that problem by adding the llvm.loop.unroll.disable metadata to only the loop which was just unrolled.

[hfinkel]

In true LLVM style, I ask for a 40-line test case for your one-line change ;)

-Hal

• Original Message -----

From: "Mark Heffernan" <[e-mail]>
To: [e-mail], [e-mail]
Cc: [e-mail]
Sent: Thursday, July 24, 2014 4:59:24 PM
Subject: [PATCH] Fix bug causing loop unrolling to be disabled in some cases for loops with '#pragma clang loop
unroll_count(N)"

Hi hfinkel,

After unrolling a loop with llvm.loop.unroll.count metadata (unroll
factor hint) the loop unroller replaces the llvm.loop.unroll.count
metadata with llvm.loop.unroll.disable metadata to prevent any
subsequent unrolling passes from unrolling more than the hint
indicates. However, if the unroll count metadata is shared between
loops then unrolling for these other loops will also be disabled.
This small patch fixes that problem by adding the
llvm.loop.unroll.disable metadata to only the loop which was just
unrolled.

http://reviews.llvm.org/D4662

Files:

lib/Transforms/Scalar/LoopUnrollPass.cpp

Index: lib/Transforms/Scalar/LoopUnrollPass.cpp

• lib/Transforms/Scalar/LoopUnrollPass.cpp

+++ lib/Transforms/Scalar/LoopUnrollPass.cpp
@@ -302,7 +302,6 @@

// Set operand 0 to refer to the loop id itself.
NewLoopID->replaceOperandWith(0, NewLoopID);
L->setLoopID(NewLoopID);

• LoopID->replaceAllUsesWith(NewLoopID); }
  
  unsigned LoopUnroll::selectUnrollCount(

[hfinkel]

• Original Message -----

From: "Hal Finkel" <[e-mail]>
To: [e-mail]
Cc: [e-mail]
Sent: Thursday, July 24, 2014 5:02:48 PM
Subject: Re: [PATCH] Fix bug causing loop unrolling to be disabled in some cases for loops with '#pragma clang loop
unroll_count(N)"

In true LLVM style, I ask for a 40-line test case for your one-line
change ;)

To be clear, otherwise, LGTM!

Thanks,
Hal

-Hal

• Original Message -----

From: "Mark Heffernan" <[e-mail]>
To: [e-mail], [e-mail]
Cc: [e-mail]
Sent: Thursday, July 24, 2014 4:59:24 PM
Subject: [PATCH] Fix bug causing loop unrolling to be disabled in
some cases for loops with '#pragma clang loop
unroll_count(N)"

Hi hfinkel,

After unrolling a loop with llvm.loop.unroll.count metadata (unroll
factor hint) the loop unroller replaces the llvm.loop.unroll.count
metadata with llvm.loop.unroll.disable metadata to prevent any
subsequent unrolling passes from unrolling more than the hint
indicates. However, if the unroll count metadata is shared between
loops then unrolling for these other loops will also be disabled.
This small patch fixes that problem by adding the
llvm.loop.unroll.disable metadata to only the loop which was just
unrolled.

http://reviews.llvm.org/D4662

Files:

lib/Transforms/Scalar/LoopUnrollPass.cpp

Index: lib/Transforms/Scalar/LoopUnrollPass.cpp

• lib/Transforms/Scalar/LoopUnrollPass.cpp

+++ lib/Transforms/Scalar/LoopUnrollPass.cpp
@@ -302,7 +302,6 @@

// Set operand 0 to refer to the loop id itself.
NewLoopID->replaceOperandWith(0, NewLoopID);
L->setLoopID(NewLoopID);

• LoopID->replaceAllUsesWith(NewLoopID); }
  
  unsigned LoopUnroll::selectUnrollCount(

Hal Finkel
Assistant Computational Scientist
Leadership Computing Facility
Argonne National Laboratory

---

llvm-commits mailing list
[e-mail]
http://lists.cs.uiuc.edu/mailman/listinfo/llvm-commits

[meheff]

Thanks! Test added. Will lead with that next time ;-)

[hfinkel]

Thanks! (LGTM, again!)

[meheff]

Committed as r213900.

Mark

lib/Transforms/Scalar/LoopUnrollPass.cpp

static void SetLoopAlreadyUnrolled(Loop *L) {
   DisableOperands.push_back(MDString::get(Context, "llvm.loop.unroll.disable"));
   MDNode *DisableNode = MDNode::get(Context, DisableOperands);
   Vals.push_back(DisableNode);
 
   MDNode *NewLoopID = MDNode::get(Context, Vals);
   // Set operand 0 to refer to the loop id itself.
   NewLoopID->replaceOperandWith(0, NewLoopID);
   L->setLoopID(NewLoopID);
-  LoopID->replaceAllUsesWith(NewLoopID);
 }
 
 unsigned LoopUnroll::selectUnrollCount(
     const Loop *L, unsigned TripCount, bool PragmaFullUnroll,
     unsigned PragmaCount, const TargetTransformInfo::UnrollingPreferences &UP,
     bool &SetExplicitly) {
   SetExplicitly = true;