[SYCL] Add barrier optimization pass #19353

MrSidims · 2025-07-09T00:09:42Z

It removes redundant barriers (both back-to-back and in general in CFG) and downgrades global barrier to local if there are no global memory accesses 'between' them. See description in
SYCLOptimizeBackToBackBarrier.cpp for more details.

llvm/lib/SYCLLowerIR/SYCLOptimizeBarriers.cpp

wenju-he · 2025-07-10T05:39:03Z

llvm/lib/SYCLLowerIR/SYCLOptimizeBarriers.cpp

+  Changed |= eliminateBoundaryBarriers(BarrierPtrs);
+  // Then remove redundant barriers within a single basic block.
+  for (auto &BarrierBBPair : BarriersByBB)
+    Changed = eliminateBackToBackInBB(BarrierBBPair.first, BarrierBBPair.second,


can eliminateBackToBackInBB be merged into eliminateDominatedBarriers? eliminateBackToBackInBB is just a special case of the latter in that all barriers are in a single BB, right?

Yes, it can be merged. Yet I've left them split as eliminate back to back barriers function is algorithmic-wise simpler, then CFG elimination. And I though that it's a good idea to first optimize back-to-back barriers, then (not yet implemented) hoist 2 or more barriers into one in case if their appropriate blocks share the same predecessor and their semantics match, and only then do CFG-aware removal/downgrade on the remaining barriers).

wenju-he · 2025-07-10T06:32:36Z

llvm/lib/SYCLLowerIR/SYCLOptimizeBarriers.cpp

+
+      // If identical then drop Cur.
+      if (CmpExec == CompareRes::EQUAL && CmpMem == CompareRes::EQUAL) {
+        if (noFencedMemAccessesBetween(Last.CI, Cur.CI, FenceLast, BBMemInfo)) {


just a note: there could be repeated classifyMemScope calculation of somes instructions in noFencedMemAccessesBetween, e.g. following case:

barrier(CrossDevice) Instruction Set 1 (RegionMemScope == None) barrier(Device) Instruction Set 2 (RegionMemScope == None) barrier(Workgroup) Instruction Set 3 (RegionMemScope == None) barrier(Subgroup)

I guess the case is rare, so probably no need to optimize.

True. But this would require to do extra memorization (by default), which might be worse comparing extra calculus in the rare case.

In general there are other ways to define fenced regions between barriers, but I haven't though about them until last Monday, when I found a similar work :) Re-making scanning and re-defining fenced regions is a possible enhancement for the pass.

In general there are other ways to define fenced regions between barriers, but I haven't though about them until last Monday, when I found a similar work :) Re-making scanning and re-defining fenced regions is a possible enhancement for the pass.

Sounding interesting, is there a link to the work?

@wenju-he I meant CPU middle end pass, which while is not doing the same as this pass, yet have quite interesting idea for function preparation :)

I see. Right it is not the same. I agree that a region based algorithm would be better. Basically the pass here is merging equivalent regions.

wenju-he · 2025-07-10T06:57:55Z

llvm/lib/SYCLLowerIR/SYCLOptimizeBarriers.cpp

+      if (Fence == RegionMemScope::Unknown)
+        continue;
+
+      if (DT.dominates(B1->CI, B2->CI)) {


is there repeated calculation for the case

B1 = A0, B2 = A1, A0 dominates A1, noFencedAccessesCFG returns false

B1 = A1, B2 = A0, A1 post-dominates A0, noFencedAccessesCFG is called again on the same instructions

another potential repeated calculation case is:
A0 dominates A1, A1 dominates A2. noFencedAccessesCFG is called twice on the instructions between A0 and A1.

Yeah, this is something I'm refactoring and will continue to refactor by merging elimination in CFG and downgrade functions.

Should be partially resolved.

It removes redundant barriers (both back-to-back and in general in CFG) and downgrades global barrier to local if there are no global memory accesses 'between' them. See description in SYCLOptimizeBackToBackBarrier.cpp for more details. Signed-off-by: Sidorov, Dmitry <[email protected]>

llvm/lib/SYCLLowerIR/SYCLOptimizeBarriers.cpp

TODO: merge CFG elimination and barrier downgrade Signed-off-by: Sidorov, Dmitry <[email protected]>

llvm/lib/SYCLLowerIR/SYCLOptimizeBarriers.cpp

clang/lib/CodeGen/BackendUtil.cpp

maarquitos14

LGTM, just one nit about a redundant comment.

Signed-off-by: Sidorov, Dmitry <[email protected]>

wenju-he

LGTM

Signed-off-by: Sidorov, Dmitry <[email protected]>

MrSidims · 2025-07-22T11:42:45Z

@intel/llvm-gatekeepers not sure if @intel/dpcpp-cfe-reviewers approval is mandatory here. If you agree, please help with the merge.

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MrSidims changed the title ~~[testing for now][SYCL] Add barrier optimization pass~~ [SYCL] Add barrier optimization pass Jul 13, 2025

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Rewrite logic for fenced memory detection and many more

1397c5b

TODO: merge CFG elimination and barrier downgrade Signed-off-by: Sidorov, Dmitry <[email protected]>

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maarquitos14 approved these changes Jul 21, 2025

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Address few changes, simplify CFG scan

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Add a test

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Signed-off-by: Sidorov, Dmitry <[email protected]>

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restore part of at exit/entry checks (no scanning though)

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format

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fix test

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Signed-off-by: Sidorov, Dmitry <[email protected]>

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DenseMap -> MapVector to solve non deterministic behaviour

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steffenlarsen merged commit 0231525 into intel:sycl Jul 22, 2025
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[SYCL] Add barrier optimization pass #19353

[SYCL] Add barrier optimization pass #19353

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