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CodeGen: BranchFolding: Merge identical blocks, even if they are short. 

Merging identical blocks when it doesn't reduce fallthrough. It is common for
the blocks created from critical edge splitting to be identical. We would like
to merge these blocks whenever doing so would not reduce fallthrough.

llvm-svn: 299890
This commit is contained in:
Kyle Butt 2017-04-10 22:28:12 +00:00
parent 502ad60c8f
commit a12bd756e4
2 changed files with 57 additions and 0 deletions
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16
llvm/lib/CodeGen/BranchFolding.cpp
View File

@ -600,6 +600,22 @@ ProfitableToMerge(MachineBasicBlock *MBB1, MachineBasicBlock *MBB2,
if (MBB2->isLayoutSuccessor(MBB1) && I1 == MBB1->begin())
return true;
// If both blocks are identical and end in a branch, merge them unless they
// both have a fallthrough predecessor and successor.
// We can only do this after block placement because it depends on whether
// there are fallthroughs, and we don't know until after layout.
if (AfterPlacement && I1 == MBB1->begin() && I2 == MBB2->begin()) {
auto BothFallThrough = [](MachineBasicBlock *MBB) {
if (MBB->succ_size() != 0 && !MBB->canFallThrough())
return false;
MachineFunction::iterator I(MBB);
MachineFunction *MF = MBB->getParent();
return (MBB != &*MF->begin()) && std::prev(I)->canFallThrough();
};
if (!BothFallThrough(MBB1) || !BothFallThrough(MBB2))
return true;
}
// If both blocks have an unconditional branch temporarily stripped out,
// count that as an additional common instruction for the following
// heuristics. This heuristic is only accurate for single-succ blocks, so to

41
llvm/test/CodeGen/X86/tail-merge-identical.ll Normal file
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@ -0,0 +1,41 @@
; RUN: llc -o - -verify-machineinstrs %s | FileCheck %s
target datalayout = "e-m:e-i64:64-f80:128-n8:16:32:64-S128"
target triple = "x86_64-unknown-linux-gnu"
@data = external global [3 x i32], align 4
@store = external global i32, align 4
; %else1 and %then2 end up lowering to identical blocks. These blocks should be
; merged during tail-merging.
; CHECK-LABEL: merge_identical_blocks
; CHECK: movl $data+4
; CHECK-NOT: movl $data+4
; CHECK: retq
define void @merge_identical_blocks(i1 %a, i1 %b) {
entry:
br label %if1
if1: ; predfs = %entry
br i1 %a, label %else1, label %if2
else1: ; preds = %if1
%ptr.else1 = getelementptr inbounds [3 x i32], [3 x i32]* @data, i64 0, i32 1
br label %phi_join
if2: ; preds = %if1
br i1 %b, label %then2, label %else2
then2: ; preds = %if2
%ptr.then2 = getelementptr inbounds [3 x i32], [3 x i32]* @data, i64 0, i32 1
br label %phi_join
else2: ; preds = %if2
%ptr.else2 = getelementptr inbounds [3 x i32], [3 x i32]* @data, i64 0, i32 2
br label %phi_join
phi_join: ; preds = %else1, %then2, %else2
%val.ptr = phi i32* [ %ptr.else1, %else1 ], [ %ptr.then2, %then2 ], [ %ptr.else2, %else2 ]
%val = load i32, i32* %val.ptr, align 4
store i32 %val, i32* @store, align 4
ret void
}