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MachineCopyPropagation has special logic for removing COPY instructions. It will remove plain COPYs using eraseFromParent(), but if the COPY has imp-defs/imp-uses it will convert it to a KILL, to keep the imp-def around. 

This actually totally breaks and causes the machine verifier to cry in several cases, one of which being:

%RAX<def> = COPY %RCX<kill>
%ECX<def> = COPY %EAX<kill>, %RAX<imp-use,kill>

These subregister copies are together identified as noops, so are both removed. However, the second one as it has an imp-use gets converted into a kill:

%ECX<def> = KILL %EAX<kill>, %RAX<imp-use,kill>

As the original COPY has been removed, the verifier goes into tears at the use of undefined EAX and RAX.

There are several hacky solutions to this hacky problem (which is all to do with imp-use/def weirdnesses), but the least hacky I've come up with is to *always* remove COPYs by converting to KILLs. KILLs are no-ops to the code generator so the generated code doesn't change (which is why they were partially used in the first place), but using them also keeps the def/use and imp-def/imp-use chains alive:

%RAX<def> = KILL %RCX<kill>
%ECX<def> = KILL %EAX<kill>, %RAX<imp-use,kill>

The patch passes all test cases including the ones that check the removal of MOVs in this circumstance, along with an extra test I added to check subregister behaviour (which made the machine verifier fall over before my patch).

The patch also adds some DEBUG() statements because the file hadn't got any.

llvm-svn: 199797
This commit is contained in:
James Molloy 2014-01-22 09:12:27 +00:00
parent 215d6c8c50
commit d787d3e593
2 changed files with 42 additions and 9 deletions
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25
llvm/lib/CodeGen/MachineCopyPropagation.cpp
View File

@ -127,13 +127,10 @@ static bool isNopCopy(MachineInstr *CopyMI, unsigned Def, unsigned Src,
}
// Remove MI from the function because it has been determined it is dead.
// Turn it into a noop KILL instruction if it has super-register liveness
// adjustments.
// Turn it into a noop KILL instruction as opposed to removing it to
// maintain imp-use/imp-def chains.
void MachineCopyPropagation::removeCopy(MachineInstr *MI) {
if (MI->getNumOperands() == 2)
MI->eraseFromParent();
else
MI->setDesc(TII->get(TargetOpcode::KILL));
MI->setDesc(TII->get(TargetOpcode::KILL));
}
bool MachineCopyPropagation::CopyPropagateBlock(MachineBasicBlock &MBB) {
@ -142,6 +139,8 @@ bool MachineCopyPropagation::CopyPropagateBlock(MachineBasicBlock &MBB) {
DenseMap<unsigned, MachineInstr*> CopyMap; // Def -> copies map
SourceMap SrcMap; // Src -> Def map
DEBUG(dbgs() << "MCP: CopyPropagateBlock " << MBB.getName() << "\n");
bool Changed = false;
for (MachineBasicBlock::iterator I = MBB.begin(), E = MBB.end(); I != E; ) {
MachineInstr *MI = &*I;
@ -176,6 +175,8 @@ bool MachineCopyPropagation::CopyPropagateBlock(MachineBasicBlock &MBB) {
// CALL
// %RAX<def> = COPY %RSP
DEBUG(dbgs() << "MCP: copy is a NOP, removing: "; MI->dump());
// Clear any kills of Def between CopyMI and MI. This extends the
// live range.
for (MachineBasicBlock::iterator I = CopyMI, E = MI; I != E; ++I)
@ -191,10 +192,14 @@ bool MachineCopyPropagation::CopyPropagateBlock(MachineBasicBlock &MBB) {
// If Src is defined by a previous copy, it cannot be eliminated.
for (MCRegAliasIterator AI(Src, TRI, true); AI.isValid(); ++AI) {
CI = CopyMap.find(*AI);
if (CI != CopyMap.end())
if (CI != CopyMap.end()) {
DEBUG(dbgs() << "MCP: Copy is no longer dead: "; CI->second->dump());
MaybeDeadCopies.remove(CI->second);
}
}
DEBUG(dbgs() << "MCP: Copy is a deletion candidate: "; MI->dump());
// Copy is now a candidate for deletion.
MaybeDeadCopies.insert(MI);
@ -255,8 +260,10 @@ bool MachineCopyPropagation::CopyPropagateBlock(MachineBasicBlock &MBB) {
// for elimination.
for (MCRegAliasIterator AI(Reg, TRI, true); AI.isValid(); ++AI) {
DenseMap<unsigned, MachineInstr*>::iterator CI = CopyMap.find(*AI);
if (CI != CopyMap.end())
if (CI != CopyMap.end()) {
DEBUG(dbgs() << "MCP: Copy is used - not dead: "; CI->second->dump());
MaybeDeadCopies.remove(CI->second);
}
}
}
@ -273,6 +280,8 @@ bool MachineCopyPropagation::CopyPropagateBlock(MachineBasicBlock &MBB) {
unsigned Reg = (*DI)->getOperand(0).getReg();
if (MRI->isReserved(Reg) || !MaskMO.clobbersPhysReg(Reg))
continue;
DEBUG(dbgs() << "MCP: Removing copy due to regmask clobbering: ";
(*DI)->dump());
removeCopy(*DI);
Changed = true;
++NumDeletes;

26
llvm/test/CodeGen/X86/machine-cp.ll
View File

@ -1,4 +1,4 @@
; RUN: llc -mtriple=x86_64-apple-macosx -mcpu=nocona < %s | FileCheck %s
; RUN: llc -mtriple=x86_64-apple-macosx -mcpu=nocona -verify-machineinstrs < %s | FileCheck %s
; After tail duplication, two copies in an early exit BB can be cancelled out.
; rdar://10640363
@ -34,3 +34,27 @@ entry:
%tmp8 = shufflevector <8 x i16> %T0, <8 x i16> %T1, <8 x i32> < i32 undef, i32 undef, i32 7, i32 2, i32 8, i32 undef, i32 undef , i32 undef >
ret <8 x i16> %tmp8
}
define i32 @t3(i64 %a, i64 %b) nounwind {
entry:
; CHECK-LABEL: t3:
; CHECK: je [[LABEL:.*BB.*]]
%cmp1 = icmp eq i64 %b, 0
br i1 %cmp1, label %while.end, label %while.body
; CHECK: [[LABEL]]:
; CHECK-NOT: mov
; CHECK: ret
while.body: ; preds = %entry, %while.body
%a.addr.03 = phi i64 [ %b.addr.02, %while.body ], [ %a, %entry ]
%b.addr.02 = phi i64 [ %rem, %while.body ], [ %b, %entry ]
%rem = srem i64 %a.addr.03, %b.addr.02
%cmp = icmp eq i64 %rem, 0
br i1 %cmp, label %while.end, label %while.body
while.end: ; preds = %while.body, %entry
%a.addr.0.lcssa = phi i64 [ %a, %entry ], [ %b.addr.02, %while.body ]
%t = trunc i64 %a.addr.0.lcssa to i32
ret i32 %t
}