[PATCH] D123865: [LoopPeel] Allow partial unrolling for profile-based peeling

[Igor Kudrin via Phabricator via llvm-commits]

ikudrin updated this revision to Diff 424899.
ikudrin added a comment.

- Extract an NFCI part into D124388 <https://reviews.llvm.org/D124388>

In D123865#3468366 <https://reviews.llvm.org/D123865#3468366>, @davidxl wrote:

> Thanks for the analysis.   That matches what I expected -- which circles back to my original question -- how do we know the new limit is generally better than the old limit?  It really depends on the cost-benefit analysis. There are workloads (as demonstrated by the Decompressing case) which gets hurt.
>
> On the other hand, the patch itself (the restructuring part) looks good -- it makes the code cleaner.   I suggest you commit the restructuring part first (e.g. by removing the *2 multiplier part).     I  think it is better to keep the default setting unchanged until a better analysis in place.

I am not sure I understand why this patch requires any additional analysis compared to what is already implemented. The existing heuristic states that extracting up to 7 iterations for loops with estimated loop counts up to 7 is beneficial. The new heuristic is that peeling the same 7 iterations for loops with estimated loop counts slightly greater than 7 is still beneficial. They are still the same 7 iterations, with exactly the same profit. If the analysis is required, it should be applied to the whole idea of peeling loops based on the estimated trip counts.


CHANGES SINCE LAST ACTION
  https://reviews.llvm.org/D123865/new/

https://reviews.llvm.org/D123865

Files:
  llvm/lib/Transforms/Utils/LoopPeel.cpp
  llvm/test/Transforms/LoopUnroll/peel-loop-conditions-pgo-2.ll
  llvm/test/Transforms/LoopUnroll/peel-loop-conditions-pgo-3.ll


Index: llvm/test/Transforms/LoopUnroll/peel-loop-conditions-pgo-3.ll
===================================================================
--- /dev/null
+++ llvm/test/Transforms/LoopUnroll/peel-loop-conditions-pgo-3.ll
@@ -0,0 +1,43 @@
+; RUN: opt < %s -S -loop-unroll -loop-unroll -verify-dom-info -debug-only=loop-unroll -unroll-peel-max-count=7 2>&1 | FileCheck %s
+; REQUIRES: asserts
+
+declare void @f1()
+declare void @f2()
+
+; Check that we can peel off iterations that make conditions true.
+; The second invocation of loop-unroll will do profile-based peeling of remaining
+; iterations and will partially peel them so that the total number of peeled
+; iterations does not exceed the threshold specified with -unroll-peel-max-count=7.
+define void @test2(i32 %k) !prof !4 {
+; CHECK: Loop Unroll: F[test2] Loop %for.body
+; CHECK: PEELING loop %for.body with iteration count 2!
+; CHECK: PEELING loop %for.body with iteration count 5!
+; CHECK: llvm.loop.unroll.disable
+for.body.lr.ph:
+  br label %for.body
+
+for.body:
+  %i.05 = phi i32 [ 0, %for.body.lr.ph ], [ %inc, %for.inc ]
+  %cmp1 = icmp ult i32 %i.05, 2
+  br i1 %cmp1, label %if.then, label %if.else
+
+if.then:
+  call void @f1()
+  br label %for.inc
+
+if.else:
+  call void @f2()
+  br label %for.inc
+
+for.inc:
+  %inc = add nsw i32 %i.05, 1
+  %cmp = icmp slt i32 %inc, %k
+  br i1 %cmp, label %for.body, label %for.end, !llvm.loop !1, !prof !3
+
+for.end:
+  ret void
+}
+
+!1 = distinct !{!1}
+!3 = !{!"branch_weights", i32 13, i32 1}
+!4 = !{!"function_entry_count", i64 1}
Index: llvm/test/Transforms/LoopUnroll/peel-loop-conditions-pgo-2.ll
===================================================================
--- llvm/test/Transforms/LoopUnroll/peel-loop-conditions-pgo-2.ll
+++ llvm/test/Transforms/LoopUnroll/peel-loop-conditions-pgo-2.ll
@@ -7,8 +7,8 @@
 
 ; Check that we can peel off iterations that make conditions true.
 ; The second invocation of loop-unroll will NOT do profile based peeling of
-; remained iterations because the total number of peeled iterations exceeds
-; threashold specified with -unroll-peel-max-count=7.
+; remaining iterations because the total number of iterations exceeds the
+; doubled threshold specified with -unroll-peel-max-count=7.
 define void @test2(i32 %k) !prof !4 {
 ; CHECK: Loop Unroll: F[test2] Loop %for.body
 ; CHECK: PEELING loop %for.body with iteration count 2!
@@ -39,5 +39,5 @@
 }
 
 !1 = distinct !{!1}
-!3 = !{!"branch_weights", i32 8, i32 1}
+!3 = !{!"branch_weights", i32 14, i32 1}
 !4 = !{!"function_entry_count", i64 1}
Index: llvm/lib/Transforms/Utils/LoopPeel.cpp
===================================================================
--- llvm/lib/Transforms/Utils/LoopPeel.cpp
+++ llvm/lib/Transforms/Utils/LoopPeel.cpp
@@ -468,8 +468,12 @@
                       << *EstimatedTripCount << "\n");
 
     if (*EstimatedTripCount) {
-      if (*EstimatedTripCount + AlreadyPeeled <= MaxPeelCount) {
-        unsigned PeelCount = *EstimatedTripCount;
+      // Even if we cannot peel all estimated iterations, partial peeling also
+      // seems beneficial, but only to some extent.
+      if (*EstimatedTripCount + AlreadyPeeled <= MaxPeelCount * 2 &&
+          AlreadyPeeled < MaxPeelCount) {
+        unsigned PeelCount =
+            std::min(*EstimatedTripCount, MaxPeelCount - AlreadyPeeled);
         LLVM_DEBUG(dbgs() << "Peeling first " << PeelCount << " iterations.\n");
         PP.PeelCount = PeelCount;
         return;


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