[BranchFolding] Tail common all identical unreachable blocks

Summary:
Blocks ending in unreachable are typically cold because they end the
program or throw an exception, so merging them with other identical
blocks is usually profitable because it reduces the size of cold code.
MachineBlockPlacement generally does not arrange to fall through to such
blocks, so commoning these blocks will not introduce additional
unconditional branches.

Reviewers: hans, iteratee, haicheng

Subscribers: llvm-commits

Differential Revision: https://reviews.llvm.org/D29153

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@295105 91177308-0d34-0410-b5e6-96231b3b80d8

diff --git a/lib/CodeGen/BranchFolding.cpp b/lib/CodeGen/BranchFolding.cpp
index b196bf98d27151c51bdedf5cb54ffec2e968a738..d148a9dbac549da793bb60d47b962402d42fafbb 100644 (file)
--- a/lib/CodeGen/BranchFolding.cpp
+++ b/lib/CodeGen/BranchFolding.cpp
@@ -523,6 +523,17 @@ static unsigned CountTerminators(MachineBasicBlock *MBB,
   return NumTerms;
 }
 
+/// A no successor, non-return block probably ends in unreachable and is cold.
+/// Also consider a block that ends in an indirect branch to be a return block,
+/// since many targets use plain indirect branches to return.
+static bool blockEndsInUnreachable(const MachineBasicBlock *MBB) {
+  if (!MBB->succ_empty())
+    return false;
+  if (MBB->empty())
+    return true;
+  return !(MBB->back().isReturn() || MBB->back().isIndirectBranch());
+}
+
 /// ProfitableToMerge - Check if two machine basic blocks have a common tail
 /// and decide if it would be profitable to merge those tails.  Return the
 /// length of the common tail and iterators to the first common instruction
@@ -577,6 +588,15 @@ ProfitableToMerge(MachineBasicBlock *MBB1, MachineBasicBlock *MBB2,
       return true;
   }
 
+  // If these are identical non-return blocks with no successors, merge them.
+  // Such blocks are typically cold calls to noreturn functions like abort, and
+  // are unlikely to become a fallthrough target after machine block placement.
+  // Tail merging these blocks is unlikely to create additional unconditional
+  // branches, and will reduce the size of this cold code.
+  if (I1 == MBB1->begin() && I2 == MBB2->begin() &&
+      blockEndsInUnreachable(MBB1) && blockEndsInUnreachable(MBB2))
+    return true;
+
   // If one of the blocks can be completely merged and happens to be in
   // a position where the other could fall through into it, merge any number
   // of instructions, because it can be done without a branch.

diff --git a/test/CodeGen/ARM/tail-opts.ll b/test/CodeGen/ARM/tail-opts.ll
index 37e9a4af3be5969d9fec79bd220d36b7297033f4..475b80b3bb070eec6d84366483e94d68e5c08df5 100644 (file)
--- a/test/CodeGen/ARM/tail-opts.ll
+++ b/test/CodeGen/ARM/tail-opts.ll
@@ -65,3 +65,55 @@ altret:
   call void @far(i32 1001)
   ret void
 }
+
+; Use alternating abort functions so that the blocks we wish to merge are not
+; layout successors during branch folding.
+
+; CHECK-LABEL: merge_alternating_aborts:
+; CHECK-NOT: _abort
+; CHECK-NOT: _alt_abort
+; CHECK: bxne lr
+; CHECK-NOT: _abort
+; CHECK-NOT: _alt_abort
+; CHECK: LBB{{.*}}:
+; CHECK: mov lr, pc
+; CHECK: b _alt_abort
+; CHECK-NOT: _abort
+; CHECK-NOT: _alt_abort
+; CHECK: LBB{{.*}}:
+; CHECK: mov lr, pc
+; CHECK: b _abort
+; CHECK-NOT: _abort
+; CHECK-NOT: _alt_abort
+
+declare void @abort()
+declare void @alt_abort()
+
+define void @merge_alternating_aborts() {
+entry:
+  %c1 = call i1 @qux()
+  br i1 %c1, label %cont1, label %abort1
+abort1:
+  call void @abort()
+  unreachable
+cont1:
+  %c2 = call i1 @qux()
+  br i1 %c2, label %cont2, label %abort2
+abort2:
+  call void @alt_abort()
+  unreachable
+cont2:
+  %c3 = call i1 @qux()
+  br i1 %c3, label %cont3, label %abort3
+abort3:
+  call void @abort()
+  unreachable
+cont3:
+  %c4 = call i1 @qux()
+  br i1 %c4, label %cont4, label %abort4
+abort4:
+  call void @alt_abort()
+  unreachable
+cont4:
+  ret void
+}

diff --git a/test/CodeGen/X86/tail-opts.ll b/test/CodeGen/X86/tail-opts.ll
index 12c90c1a5fa97ce86c21b9adc52562dbb6f1f2e1..e7797426d891b23e1cb35c468415540abeae6cb2 100644 (file)
--- a/test/CodeGen/X86/tail-opts.ll
+++ b/test/CodeGen/X86/tail-opts.ll
@@ -299,33 +299,35 @@ declare void @func()
 ; one - One instruction may be tail-duplicated even with optsize.
 
 ; CHECK-LABEL: one:
-; CHECK: movl $0, XYZ(%rip)
-; CHECK: movl $0, XYZ(%rip)
+; CHECK: j{{.*}} tail_call_me
+; CHECK: j{{.*}} tail_call_me
 
 @XYZ = external global i32
 
-define void @one() nounwind optsize {
+declare void @tail_call_me()
+
+define void @one(i32 %v) nounwind optsize {
 entry:
-  %0 = icmp eq i32 undef, 0
+  %0 = icmp eq i32 %v, 0
   br i1 %0, label %bbx, label %bby
 
 bby:
-  switch i32 undef, label %bb7 [
+  switch i32 %v, label %bb7 [
     i32 16, label %return
   ]
 
 bb7:
-  store volatile i32 0, i32* @XYZ
-  unreachable
+  tail call void @tail_call_me()
+  ret void
 
 bbx:
-  switch i32 undef, label %bb12 [
+  switch i32 %v, label %bb12 [
     i32 128, label %return
   ]
 
 bb12:
-  store volatile i32 0, i32* @XYZ
-  unreachable
+  tail call void @tail_call_me()
+  ret void
 
 return:
   ret void
@@ -414,9 +416,9 @@ return:
 
 ; CHECK-LABEL: two_nosize:
 ; CHECK: movl $0, XYZ(%rip)
-; CHECK: movl $1, XYZ(%rip)
+; CHECK: jmp tail_call_me
 ; CHECK: movl $0, XYZ(%rip)
-; CHECK: movl $1, XYZ(%rip)
+; CHECK: jmp tail_call_me
 
 define void @two_nosize() nounwind {
 entry:
@@ -430,8 +432,8 @@ bby:
 
 bb7:
   store volatile i32 0, i32* @XYZ
-  store volatile i32 1, i32* @XYZ
-  unreachable
+  tail call void @tail_call_me()
+  ret void
 
 bbx:
   switch i32 undef, label %bb12 [
@@ -440,8 +442,8 @@ bbx:
 
 bb12:
   store volatile i32 0, i32* @XYZ
-  store volatile i32 1, i32* @XYZ
-  unreachable
+  tail call void @tail_call_me()
+  ret void
 
 return:
   ret void
@@ -469,3 +471,88 @@ bb.nph:                                           ; preds = %entry
 for.end:                                          ; preds = %entry
   ret i64 %varx.0
 }
+
+; We should tail merge small blocks that don't end in a tail call or return
+; instruction. Those blocks are typically unreachable and will be placed
+; out-of-line after the main return, so we should try to eliminate as many of
+; them as possible.
+
+; CHECK-LABEL: merge_aborts:
+; CHECK-NOT: callq abort
+; CHECK: ret
+; CHECK: callq abort
+; CHECK-NOT: callq abort
+; CHECK: .Lfunc_end{{.*}}:
+
+declare void @abort()
+define void @merge_aborts() {
+entry:
+  %c1 = call i1 @qux()
+  br i1 %c1, label %cont1, label %abort1
+abort1:
+  call void @abort()
+  unreachable
+cont1:
+  %c2 = call i1 @qux()
+  br i1 %c2, label %cont2, label %abort2
+abort2:
+  call void @abort()
+  unreachable
+cont2:
+  %c3 = call i1 @qux()
+  br i1 %c3, label %cont3, label %abort3
+abort3:
+  call void @abort()
+  unreachable
+cont3:
+  %c4 = call i1 @qux()
+  br i1 %c4, label %cont4, label %abort4
+abort4:
+  call void @abort()
+  unreachable
+cont4:
+  ret void
+}
+
+; Use alternating abort functions so that the blocks we wish to merge are not
+; layout successors during branch folding.
+
+; CHECK-LABEL: merge_alternating_aborts:
+; CHECK-NOT: callq abort
+; CHECK: ret
+; CHECK: callq abort
+; CHECK: callq alt_abort
+; CHECK-NOT: callq abort
+; CHECK-NOT: callq alt_abort
+; CHECK: .Lfunc_end{{.*}}:
+
+declare void @alt_abort()
+
+define void @merge_alternating_aborts() {
+entry:
+  %c1 = call i1 @qux()
+  br i1 %c1, label %cont1, label %abort1
+abort1:
+  call void @abort()
+  unreachable
+cont1:
+  %c2 = call i1 @qux()
+  br i1 %c2, label %cont2, label %abort2
+abort2:
+  call void @alt_abort()
+  unreachable
+cont2:
+  %c3 = call i1 @qux()
+  br i1 %c3, label %cont3, label %abort3
+abort3:
+  call void @abort()
+  unreachable
+cont3:
+  %c4 = call i1 @qux()
+  br i1 %c4, label %cont4, label %abort4
+abort4:
+  call void @alt_abort()
+  unreachable
+cont4:
+  ret void
+}