[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

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@306272 91177308-0d34-0410-b5e6-96231b3b80d8

diff --git a/lib/CodeGen/MachineBlockPlacement.cpp b/lib/CodeGen/MachineBlockPlacement.cpp
index 2d4b959..c8f8dda 100644 (file)
--- a/lib/CodeGen/MachineBlockPlacement.cpp
+++ b/lib/CodeGen/MachineBlockPlacement.cpp
@@ -1944,6 +1944,35 @@ void MachineBlockPlacement::rotateLoop(BlockChain &LoopChain,
   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());
 }
 

diff --git a/test/CodeGen/X86/block-placement.ll b/test/CodeGen/X86/block-placement.ll
index c7de65d..1975b03 100644 (file)
--- a/test/CodeGen/X86/block-placement.ll
+++ b/test/CodeGen/X86/block-placement.ll
@@ -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:                                              ; preds = %endif, %then
   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}

diff --git a/test/CodeGen/X86/code_placement_cold_loop_blocks.ll b/test/CodeGen/X86/code_placement_cold_loop_blocks.ll
index d7dc8de..875d791 100644 (file)
--- a/test/CodeGen/X86/code_placement_cold_loop_blocks.ll
+++ b/test/CodeGen/X86/code_placement_cold_loop_blocks.ll
@@ -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()