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[InlineSpiller] Don't spill fully undef values

Althought it is not wrong to spill undef values, it is useless and harms
both code size and runtime. Before spilling a value, check that its
content actually matters.

http://www.llvm.org/PR33311

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@304752 91177308-0d34-0410-b5e6-96231b3b80d8
This commit is contained in:
Quentin Colombet 2017-06-05 23:51:27 +00:00
parent 17e210d01a
commit de1ece9ed1
2 changed files with 91 additions and 2 deletions
lib/CodeGen
test/CodeGen/AArch64

@ -857,14 +857,36 @@ void InlineSpiller::insertReload(unsigned NewVReg,
++NumReloads;
}
/// Check if \p Def fully defines a VReg with an undefined value.
/// If that's the case, that means the value of VReg is actually
/// not relevant.
static bool isFullUndefDef(const MachineInstr &Def) {
if (!Def.isImplicitDef())
return false;
assert(Def.getNumOperands() == 1 &&
"Implicit def with more than one definition");
// We can say that the VReg defined by Def is undef, only if it is
// fully defined by Def. Otherwise, some of the lanes may not be
// undef and the value of the VReg matters.
return !Def.getOperand(0).getSubReg();
}
/// insertSpill - Insert a spill of NewVReg after MI.
void InlineSpiller::insertSpill(unsigned NewVReg, bool isKill,
MachineBasicBlock::iterator MI) {
MachineBasicBlock &MBB = *MI->getParent();
MachineInstrSpan MIS(MI);
TII.storeRegToStackSlot(MBB, std::next(MI), NewVReg, isKill, StackSlot,
MRI.getRegClass(NewVReg), &TRI);
if (isFullUndefDef(*MI))
// Don't spill undef value.
// Anything works for undef, in particular keeping the memory
// uninitialized is a viable option and it saves code size and
// run time.
BuildMI(MBB, std::next(MI), MI->getDebugLoc(), TII.get(TargetOpcode::KILL))
.addReg(NewVReg, getKillRegState(isKill));
else
TII.storeRegToStackSlot(MBB, std::next(MI), NewVReg, isKill, StackSlot,
MRI.getRegClass(NewVReg), &TRI);
LIS.InsertMachineInstrRangeInMaps(std::next(MI), MIS.end());

@ -0,0 +1,67 @@
# RUN: llc %s -run-pass greedy -o - | FileCheck %s
# Check that we don't insert spill code for undef values.
# Uninitialized memory for them is fine.
# PR33311
--- |
; ModuleID = 'stuff.ll'
target triple = "aarch64--"
@g = external global i32
define void @foobar() {
ret void
}
...
---
name: foobar
alignment: 2
tracksRegLiveness: true
registers:
- { id: 0, class: gpr32 }
- { id: 1, class: gpr32 }
- { id: 2, class: gpr32all }
- { id: 3, class: gpr32 }
- { id: 4, class: gpr64common }
- { id: 5, class: gpr32 }
- { id: 6, class: gpr64common }
- { id: 7, class: gpr32 }
- { id: 8, class: gpr32 }
- { id: 9, class: gpr64 }
body: |
bb.0:
liveins: %x0
successors: %bb.1, %bb.2
; %8 is going to be spilled.
; But on that path, we don't care about its value.
; Emit a simple KILL instruction instead of an
; actual spill.
; CHECK: [[UNDEF:%[0-9]+]] = IMPLICIT_DEF
; CHECK-NEXT: KILL [[UNDEF]]
%8 = IMPLICIT_DEF
; %9 us going to be spilled.
; But it is only partially undef.
; Make sure we spill it properly
; CHECK: [[NINE:%[0-9]+]] = COPY %x0
; CHECK: [[NINE]].sub_32 = IMPLICIT_DEF
; CHECK-NEXT: STRXui [[NINE]]
%9 = COPY %x0
%9.sub_32 = IMPLICIT_DEF
CBNZW %wzr, %bb.2
B %bb.1
bb.1:
%4 = ADRP target-flags(aarch64-page) @g
%8 = LDRWui %4, target-flags(aarch64-pageoff, aarch64-nc) @g :: (volatile dereferenceable load 4 from @g)
INLINEASM $nop, 1, 12, implicit-def dead early-clobber %x0, 12, implicit-def dead early-clobber %x1, 12, implicit-def dead early-clobber %x2, 12, implicit-def dead early-clobber %x3, 12, implicit-def dead early-clobber %x4, 12, implicit-def dead early-clobber %x5, 12, implicit-def dead early-clobber %x6, 12, implicit-def dead early-clobber %x7, 12, implicit-def dead early-clobber %x8, 12, implicit-def dead early-clobber %x9, 12, implicit-def dead early-clobber %x10, 12, implicit-def dead early-clobber %x11, 12, implicit-def dead early-clobber %x12, 12, implicit-def dead early-clobber %x13, 12, implicit-def dead early-clobber %x14, 12, implicit-def dead early-clobber %x15, 12, implicit-def dead early-clobber %x16, 12, implicit-def dead early-clobber %x17, 12, implicit-def dead early-clobber %x18, 12, implicit-def dead early-clobber %x19, 12, implicit-def dead early-clobber %x20, 12, implicit-def dead early-clobber %x21, 12, implicit-def dead early-clobber %x22, 12, implicit-def dead early-clobber %x23, 12, implicit-def dead early-clobber %x24, 12, implicit-def dead early-clobber %x25, 12, implicit-def dead early-clobber %x26, 12, implicit-def dead early-clobber %x27, 12, implicit-def dead early-clobber %x28, 12, implicit-def dead early-clobber %fp, 12, implicit-def dead early-clobber %lr
bb.2:
INLINEASM $nop, 1, 12, implicit-def dead early-clobber %x0, 12, implicit-def dead early-clobber %x1, 12, implicit-def dead early-clobber %x2, 12, implicit-def dead early-clobber %x3, 12, implicit-def dead early-clobber %x4, 12, implicit-def dead early-clobber %x5, 12, implicit-def dead early-clobber %x6, 12, implicit-def dead early-clobber %x7, 12, implicit-def dead early-clobber %x8, 12, implicit-def dead early-clobber %x9, 12, implicit-def dead early-clobber %x10, 12, implicit-def dead early-clobber %x11, 12, implicit-def dead early-clobber %x12, 12, implicit-def dead early-clobber %x13, 12, implicit-def dead early-clobber %x14, 12, implicit-def dead early-clobber %x15, 12, implicit-def dead early-clobber %x16, 12, implicit-def dead early-clobber %x17, 12, implicit-def dead early-clobber %x18, 12, implicit-def dead early-clobber %x19, 12, implicit-def dead early-clobber %x20, 12, implicit-def dead early-clobber %x21, 12, implicit-def dead early-clobber %x22, 12, implicit-def dead early-clobber %x23, 12, implicit-def dead early-clobber %x24, 12, implicit-def dead early-clobber %x25, 12, implicit-def dead early-clobber %x26, 12, implicit-def dead early-clobber %x27, 12, implicit-def dead early-clobber %x28, 12, implicit-def dead early-clobber %fp, 12, implicit-def dead early-clobber %lr
%6 = ADRP target-flags(aarch64-page) @g
%w0 = MOVi32imm 42
STRWui %8, %6, target-flags(aarch64-pageoff, aarch64-nc) @g :: (volatile store 4 into @g)
STRXui %9, %6, target-flags(aarch64-pageoff, aarch64-nc) @g :: (volatile store 8 into @g)
RET_ReallyLR implicit killed %w0
...