[llvm] r306272 - [MBP] do not rotate loop if it creates extra branch

Serguei Katkov via llvm-commits llvm-commits at lists.llvm.org
Sun Jun 25 23:51:09 PDT 2017


In progress of reverting, will investigate it after revert.

Thank you,
Serguei.

From: Chandler Carruth [mailto:chandlerc at gmail.com]
Sent: Monday, June 26, 2017 1:50 PM
To: Serguei Katkov <serguei.katkov at azul.com>; llvm-commits at lists.llvm.org
Subject: Re: [llvm] r306272 - [MBP] do not rotate loop if it creates extra branch

I have a strong suspicion this caused:
http://lab.llvm.org:8011/builders/clang-ppc64be-linux-multistage/builds/4273

And several other stage2 failures in the bots right now. (stack trace looks like MBP)

On Sun, Jun 25, 2017 at 10:27 PM Serguei Katkov via llvm-commits <llvm-commits at lists.llvm.org<mailto:llvm-commits at lists.llvm.org>> wrote:
Author: skatkov
Date: Sun Jun 25 22:27:27 2017
New Revision: 306272

URL: http://llvm.org/viewvc/llvm-project?rev=306272&view=rev
Log:
[MBP] do not rotate loop if it creates extra branch

This is a last fix for the corner case of PR32214. Actually this is not really corner case in general.

We should not do a loop rotation if we create an additional branch due to it.
Consider the case where we have a loop chain H, M, B, C , where
H is header with viable fallthrough from pre-header and exit from the loop
M - some middle block
B - backedge to Header but with exit from the loop also.
C - some cold block of the loop.

Let's H is determined as a best exit. If we do a loop rotation M, B, C, H we can introduce the extra branch.
Let's compute the change in number of branches:
+1 branch from pre-header to header
-1 branch from header to exit
+1 branch from header to middle block if there is such
-1 branch from cold bock to header if there is one

So if C is not a predecessor of H then we introduce extra branch.

This change actually prohibits rotation of the loop if both true
1) Best Exit has next element in chain as successor.
2) Last element in chain is not a predecessor of first element of chain.

Reviewers: iteratee, xur
Reviewed By: iteratee
Subscribers: llvm-commits
Differential Revision: https://reviews.llvm.org/D34271

Modified:
    llvm/trunk/lib/CodeGen/MachineBlockPlacement.cpp
    llvm/trunk/test/CodeGen/X86/block-placement.ll
    llvm/trunk/test/CodeGen/X86/code_placement_cold_loop_blocks.ll

Modified: llvm/trunk/lib/CodeGen/MachineBlockPlacement.cpp
URL: http://llvm.org/viewvc/llvm-project/llvm/trunk/lib/CodeGen/MachineBlockPlacement.cpp?rev=306272&r1=306271&r2=306272&view=diff
==============================================================================
--- llvm/trunk/lib/CodeGen/MachineBlockPlacement.cpp (original)
+++ llvm/trunk/lib/CodeGen/MachineBlockPlacement.cpp Sun Jun 25 22:27:27 2017
@@ -1944,6 +1944,35 @@ void MachineBlockPlacement::rotateLoop(B
   if (ExitIt == LoopChain.end())
     return;

+  // Rotating a loop exit to the bottom when there is a fallthrough to top
+  // trades the entry fallthrough for an exit fallthrough.
+  // If there is no bottom->top edge, but the chosen exit block does have
+  // a fallthrough, we break that fallthrough for nothing in return.
+
+  // Let's consider an example. We have a built chain of basic blocks
+  // B1, B2, ..., Bn, where Bk is a ExitingBB - chosen exit block.
+  // By doing a rotation we get
+  // Bk+1, ..., Bn, B1, ..., Bk
+  // Break of fallthrough to B1 is compensated by a fallthrough from Bk.
+  // If we had a fallthrough Bk -> Bk+1 it is broken now.
+  // It might be compensated by fallthrough Bn -> B1.
+  // So we have a condition to avoid creation of extra branch by loop rotation.
+  // All below must be true to avoid loop rotation:
+  //   If there is a fallthrough to top (B1)
+  //   There was fallthrough from chosen exit block (Bk) to next one (Bk+1)
+  //   There is no fallthrough from bottom (Bn) to top (B1).
+  // Please note that there is no exit fallthrough from Bn because we checked it
+  // above.
+  if (ViableTopFallthrough) {
+    MachineBasicBlock *NextBlockInChain = *std::next(ExitIt);
+    MachineBasicBlock *Bottom = *std::prev(LoopChain.end());
+    if (NextBlockInChain->isPredecessor(ExitingBB))
+      if (!Top->isPredecessor(Bottom))
+        return;
+  }
+
+  DEBUG(dbgs() << "Rotating loop to put exit " << getBlockName(ExitingBB)
+               << " at bottom\n");
   std::rotate(LoopChain.begin(), std::next(ExitIt), LoopChain.end());
 }


Modified: llvm/trunk/test/CodeGen/X86/block-placement.ll
URL: http://llvm.org/viewvc/llvm-project/llvm/trunk/test/CodeGen/X86/block-placement.ll?rev=306272&r1=306271&r2=306272&view=diff
==============================================================================
--- llvm/trunk/test/CodeGen/X86/block-placement.ll (original)
+++ llvm/trunk/test/CodeGen/X86/block-placement.ll Sun Jun 25 22:27:27 2017
@@ -354,6 +354,7 @@ define void @unnatural_cfg2() {
 ; single-source GCC.
 ; CHECK-LABEL: unnatural_cfg2
 ; CHECK: %entry
+; CHECK: %loop.header
 ; CHECK: %loop.body1
 ; CHECK: %loop.body2
 ; CHECK: %loop.body4
@@ -361,7 +362,6 @@ define void @unnatural_cfg2() {
 ; CHECK: %loop.inner2.begin
 ; CHECK: %loop.body3
 ; CHECK: %loop.inner1.begin
-; CHECK: %loop.header
 ; CHECK: %bail

 entry:
@@ -1491,6 +1491,54 @@ ret:
   ret void
 }

+define i32 @not_rotate_if_extra_branch(i32 %count) {
+; Test checks that there is no loop rotation
+; if it introduces extra branch.
+; Specifically in this case because best exit is .header
+; but it has fallthrough to .middle block and last block in
+; loop chain .slow does not have afallthrough to .header.
+; CHECK-LABEL: not_rotate_if_extra_branch
+; CHECK: %.entry
+; CHECK: %.header
+; CHECK: %.middle
+; CHECK: %.backedge
+; CHECK: %.slow
+; CHECK: %.bailout
+; CHECK: %.stop
+.entry:
+  %sum.0 = shl nsw i32 %count, 1
+  br label %.header
+
+.header:
+  %i = phi i32 [ %i.1, %.backedge ], [ 0, %.entry ]
+  %sum = phi i32 [ %sum.1, %.backedge ], [ %sum.0, %.entry ]
+  %is_exc = icmp sgt i32 %i, 9000000
+  br i1 %is_exc, label %.bailout, label %.middle, !prof !13
+
+.bailout:
+  %sum.2 = add nsw i32 %count, 1
+  br label %.stop
+
+.middle:
+  %pr.1 = and i32 %i, 1023
+  %pr.2 = icmp eq i32 %pr.1, 0
+  br i1 %pr.2, label %.slow, label %.backedge, !prof !14
+
+.slow:
+  tail call void @effect(i32 %sum)
+  br label %.backedge
+
+.backedge:
+  %sum.1 = add nsw i32 %i, %sum
+  %i.1 = add nsw i32 %i, 1
+  %end = icmp slt i32 %i.1, %count
+  br i1 %end, label %.header, label %.stop, !prof !15
+
+.stop:
+  %sum.phi = phi i32 [ %sum.1, %.backedge ], [ %sum.2, %.bailout ]
+  ret i32 %sum.phi
+}
+
 declare void @effect(i32)

 !5 = !{!"branch_weights", i32 84, i32 16}
@@ -1501,3 +1549,6 @@ declare void @effect(i32)
 !10 = !{!"branch_weights", i32 90, i32 10}
 !11 = !{!"branch_weights", i32 1, i32 1}
 !12 = !{!"branch_weights", i32 5, i32 3}
+!13 = !{!"branch_weights", i32 1, i32 1}
+!14 = !{!"branch_weights", i32 1, i32 1023}
+!15 = !{!"branch_weights", i32 4095, i32 1}

Modified: llvm/trunk/test/CodeGen/X86/code_placement_cold_loop_blocks.ll
URL: http://llvm.org/viewvc/llvm-project/llvm/trunk/test/CodeGen/X86/code_placement_cold_loop_blocks.ll?rev=306272&r1=306271&r2=306272&view=diff
==============================================================================
--- llvm/trunk/test/CodeGen/X86/code_placement_cold_loop_blocks.ll (original)
+++ llvm/trunk/test/CodeGen/X86/code_placement_cold_loop_blocks.ll Sun Jun 25 22:27:27 2017
@@ -37,7 +37,7 @@ end:
   ret void
 }

-define void @nested_loop_0() !prof !1 {
+define void @nested_loop_0(i1 %flag) !prof !1 {
 ; Test if a block that is cold in the inner loop but not cold in the outer loop
 ; will merged to the outer loop chain.
 ;
@@ -68,8 +68,7 @@ if.then:

 if.else:
   call void @e()
-  %call2 = call zeroext i1 @a()
-  br i1 %call2, label %header2, label %header, !prof !3
+  br i1 %flag, label %header2, label %header, !prof !3

 end:
   call void @f()


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