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[BranchFolding] Tail common all identical unreachable blocks

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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

llvm-svn: 295105
This commit is contained in:
Reid Kleckner 2017-02-14 21:02:24 +00:00
parent 682e0f7f2b
commit 96b6dea648
3 changed files with 175 additions and 16 deletions
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20
lib/CodeGen/BranchFolding.cpp
View File

@ -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.

52
test/CodeGen/ARM/tail-opts.ll
View File

@ -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
}

119
test/CodeGen/X86/tail-opts.ll
View File

@ -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
}