[llvm-commits] [llvm] r161409 - in /llvm/trunk: lib/CodeGen/MachineBlockPlacement.cpp test/CodeGen/X86/2008-10-27-CoalescerBug.ll test/CodeGen/X86/block-placement.ll test/CodeGen/X86/loop-blocks.ll test/Transforms/LoopStrengthReduce/X86/2012-01-13-phielim.ll
Chandler Carruth
chandlerc at gmail.com
Tue Aug 7 02:45:24 PDT 2012
Author: chandlerc
Date: Tue Aug 7 04:45:24 2012
New Revision: 161409
URL: http://llvm.org/viewvc/llvm-project?rev=161409&view=rev
Log:
Add a much more conservative strategy for aligning branch targets.
Previously, MBP essentially aligned every branch target it could. This
bloats code quite a bit, especially non-looping code which has no real
reason to prefer aligned branch targets so heavily.
As Andy said in review, it's still a bit odd to do this without a real
cost model, but this at least has much more plausible heuristics.
Fixes PR13265.
Modified:
llvm/trunk/lib/CodeGen/MachineBlockPlacement.cpp
llvm/trunk/test/CodeGen/X86/2008-10-27-CoalescerBug.ll
llvm/trunk/test/CodeGen/X86/block-placement.ll
llvm/trunk/test/CodeGen/X86/loop-blocks.ll
llvm/trunk/test/Transforms/LoopStrengthReduce/X86/2012-01-13-phielim.ll
Modified: llvm/trunk/lib/CodeGen/MachineBlockPlacement.cpp
URL: http://llvm.org/viewvc/llvm-project/llvm/trunk/lib/CodeGen/MachineBlockPlacement.cpp?rev=161409&r1=161408&r2=161409&view=diff
==============================================================================
--- llvm/trunk/lib/CodeGen/MachineBlockPlacement.cpp (original)
+++ llvm/trunk/lib/CodeGen/MachineBlockPlacement.cpp Tue Aug 7 04:45:24 2012
@@ -1011,29 +1011,63 @@
// Walk through the backedges of the function now that we have fully laid out
// the basic blocks and align the destination of each backedge. We don't rely
- // on the loop info here so that we can align backedges in unnatural CFGs and
- // backedges that were introduced purely because of the loop rotations done
- // during this layout pass.
- // FIXME: This isn't quite right, we shouldn't align backedges that result
- // from blocks being sunken below the exit block for the function.
+ // exclusively on the loop info here so that we can align backedges in
+ // unnatural CFGs and backedges that were introduced purely because of the
+ // loop rotations done during this layout pass.
if (F.getFunction()->hasFnAttr(Attribute::OptimizeForSize))
return;
unsigned Align = TLI->getPrefLoopAlignment();
if (!Align)
return; // Don't care about loop alignment.
+ if (FunctionChain.begin() == FunctionChain.end())
+ return; // Empty chain.
- SmallPtrSet<MachineBasicBlock *, 16> PreviousBlocks;
- for (BlockChain::iterator BI = FunctionChain.begin(),
+ const BranchProbability ColdProb(1, 5); // 20%
+ BlockFrequency EntryFreq = MBFI->getBlockFreq(F.begin());
+ BlockFrequency WeightedEntryFreq = EntryFreq * ColdProb;
+ for (BlockChain::iterator BI = llvm::next(FunctionChain.begin()),
BE = FunctionChain.end();
BI != BE; ++BI) {
- PreviousBlocks.insert(*BI);
- // Set alignment on the destination of all the back edges in the new
- // ordering.
- for (MachineBasicBlock::succ_iterator SI = (*BI)->succ_begin(),
- SE = (*BI)->succ_end();
- SI != SE; ++SI)
- if (PreviousBlocks.count(*SI))
- (*SI)->setAlignment(Align);
+ // Don't align non-looping basic blocks. These are unlikely to execute
+ // enough times to matter in practice. Note that we'll still handle
+ // unnatural CFGs inside of a natural outer loop (the common case) and
+ // rotated loops.
+ MachineLoop *L = MLI->getLoopFor(*BI);
+ if (!L)
+ continue;
+
+ // If the block is cold relative to the function entry don't waste space
+ // aligning it.
+ BlockFrequency Freq = MBFI->getBlockFreq(*BI);
+ if (Freq < WeightedEntryFreq)
+ continue;
+
+ // If the block is cold relative to its loop header, don't align it
+ // regardless of what edges into the block exist.
+ MachineBasicBlock *LoopHeader = L->getHeader();
+ BlockFrequency LoopHeaderFreq = MBFI->getBlockFreq(LoopHeader);
+ if (Freq < (LoopHeaderFreq * ColdProb))
+ continue;
+
+ // Check for the existence of a non-layout predecessor which would benefit
+ // from aligning this block.
+ MachineBasicBlock *LayoutPred = *llvm::prior(BI);
+
+ // Force alignment if all the predecessors are jumps. We already checked
+ // that the block isn't cold above.
+ if (!LayoutPred->isSuccessor(*BI)) {
+ (*BI)->setAlignment(Align);
+ continue;
+ }
+
+ // Align this block if the layout predecessor's edge into this block is
+ // cold relative to the block. When this is true, othe predecessors make up
+ // all of the hot entries into the block and thus alignment is likely to be
+ // important.
+ BranchProbability LayoutProb = MBPI->getEdgeProbability(LayoutPred, *BI);
+ BlockFrequency LayoutEdgeFreq = MBFI->getBlockFreq(LayoutPred) * LayoutProb;
+ if (LayoutEdgeFreq <= (Freq * ColdProb))
+ (*BI)->setAlignment(Align);
}
}
Modified: llvm/trunk/test/CodeGen/X86/2008-10-27-CoalescerBug.ll
URL: http://llvm.org/viewvc/llvm-project/llvm/trunk/test/CodeGen/X86/2008-10-27-CoalescerBug.ll?rev=161409&r1=161408&r2=161409&view=diff
==============================================================================
--- llvm/trunk/test/CodeGen/X86/2008-10-27-CoalescerBug.ll (original)
+++ llvm/trunk/test/CodeGen/X86/2008-10-27-CoalescerBug.ll Tue Aug 7 04:45:24 2012
@@ -17,8 +17,7 @@
; CHECK: %bb30.loopexit
; CHECK: divsd %xmm0
; CHECK: movsd %xmm0, 16(%esp)
-; CHECK: .align
-; CHECK-NEXT: %bb3
+; CHECK: %bb3
bb3: ; preds = %bb30.loopexit, %bb25, %bb3
%2 = load i32* null, align 4 ; <i32> [#uses=1]
%3 = mul i32 %2, 0 ; <i32> [#uses=1]
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=161409&r1=161408&r2=161409&view=diff
==============================================================================
--- llvm/trunk/test/CodeGen/X86/block-placement.ll (original)
+++ llvm/trunk/test/CodeGen/X86/block-placement.ll Tue Aug 7 04:45:24 2012
@@ -7,10 +7,15 @@
; that is not expected to run.
; CHECK: test_ifchains:
; CHECK: %entry
+; CHECK-NOT: .align
; CHECK: %else1
+; CHECK-NOT: .align
; CHECK: %else2
+; CHECK-NOT: .align
; CHECK: %else3
+; CHECK-NOT: .align
; CHECK: %else4
+; CHECK-NOT: .align
; CHECK: %exit
; CHECK: %then1
; CHECK: %then2
@@ -76,8 +81,11 @@
; Check that we sink cold loop blocks after the hot loop body.
; CHECK: test_loop_cold_blocks:
; CHECK: %entry
+; CHECK-NOT: .align
; CHECK: %unlikely1
+; CHECK-NOT: .align
; CHECK: %unlikely2
+; CHECK: .align
; CHECK: %body1
; CHECK: %body2
; CHECK: %body3
Modified: llvm/trunk/test/CodeGen/X86/loop-blocks.ll
URL: http://llvm.org/viewvc/llvm-project/llvm/trunk/test/CodeGen/X86/loop-blocks.ll?rev=161409&r1=161408&r2=161409&view=diff
==============================================================================
--- llvm/trunk/test/CodeGen/X86/loop-blocks.ll (original)
+++ llvm/trunk/test/CodeGen/X86/loop-blocks.ll Tue Aug 7 04:45:24 2012
@@ -41,7 +41,6 @@
; CHECK-NEXT: align
; CHECK-NEXT: .LBB1_4:
; CHECK-NEXT: callq bar99
-; CHECK-NEXT: align
; CHECK-NEXT: .LBB1_1:
; CHECK-NEXT: callq body
@@ -79,7 +78,6 @@
; CHECK-NEXT: .LBB2_5:
; CHECK-NEXT: callq block_a_true_func
; CHECK-NEXT: callq block_a_merge_func
-; CHECK-NEXT: align
; CHECK-NEXT: .LBB2_1:
; CHECK-NEXT: callq body
;
@@ -139,13 +137,13 @@
; CHECK-NEXT: align
; CHECK-NEXT: .LBB3_7:
; CHECK-NEXT: callq bar100
-; CHECK-NEXT: align
; CHECK-NEXT: .LBB3_1:
; CHECK-NEXT: callq loop_header
; CHECK: jl .LBB3_7
; CHECK: jge .LBB3_3
; CHECK-NEXT: callq bar101
; CHECK-NEXT: jmp .LBB3_1
+; CHECK-NEXT: align
; CHECK-NEXT: .LBB3_3:
; CHECK: jge .LBB3_4
; CHECK-NEXT: callq bar102
Modified: llvm/trunk/test/Transforms/LoopStrengthReduce/X86/2012-01-13-phielim.ll
URL: http://llvm.org/viewvc/llvm-project/llvm/trunk/test/Transforms/LoopStrengthReduce/X86/2012-01-13-phielim.ll?rev=161409&r1=161408&r2=161409&view=diff
==============================================================================
--- llvm/trunk/test/Transforms/LoopStrengthReduce/X86/2012-01-13-phielim.ll (original)
+++ llvm/trunk/test/Transforms/LoopStrengthReduce/X86/2012-01-13-phielim.ll Tue Aug 7 04:45:24 2012
@@ -99,7 +99,6 @@
; CHECK: %for.body3.lr.ph.us.i.loopexit
; CHECK-NEXT: in Loop: Header
; CHECK-NEXT: incq
-; CHECK-NEXT: .align
; CHECK-NEXT: %for.body3.us.i
; CHECK-NEXT: Inner Loop
; CHECK: testb
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