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author | Jakob Stoklund Olesen <stoklund@2pi.dk> | 2012-03-27 15:13:58 +0000 |
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committer | Jakob Stoklund Olesen <stoklund@2pi.dk> | 2012-03-27 15:13:58 +0000 |
commit | aba6559370c3d453588103fb667ffa3b11b76652 (patch) | |
tree | ceba9ebeb8cd5280b28d1b5f28d8a2a25f2c35af | |
parent | 9c55f5965ba337469466fafec364b0ea6eca459f (diff) | |
download | llvm-aba6559370c3d453588103fb667ffa3b11b76652.tar.gz llvm-aba6559370c3d453588103fb667ffa3b11b76652.tar.bz2 llvm-aba6559370c3d453588103fb667ffa3b11b76652.tar.xz |
Add an MRI::tracksLiveness() flag.
Late optimization passes like branch folding and tail duplication can
transform the machine code in a way that makes it expensive to keep the
register liveness information up to date. There is a fuzzy line between
register allocation and late scheduling where the liveness information
degrades.
The MRI::tracksLiveness() flag makes the line clear: While true,
liveness information is accurate, and can be used for register
scavenging. Once the flag is false, liveness information is not
accurate, and can only be used as a hint.
Late passes generally don't need the liveness information, but they will
sometimes use the register scavenger to help update it. The scavenger
enforces strict correctness, and we have to spend a lot of code to
update register liveness that may never be used.
git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@153511 91177308-0d34-0410-b5e6-96231b3b80d8
-rw-r--r-- | include/llvm/CodeGen/MachineRegisterInfo.h | 22 | ||||
-rw-r--r-- | lib/CodeGen/MachineRegisterInfo.cpp | 2 | ||||
-rw-r--r-- | lib/CodeGen/RegisterScavenging.cpp | 5 |
3 files changed, 28 insertions, 1 deletions
diff --git a/include/llvm/CodeGen/MachineRegisterInfo.h b/include/llvm/CodeGen/MachineRegisterInfo.h index 69ce588a3e..3272fbd78f 100644 --- a/include/llvm/CodeGen/MachineRegisterInfo.h +++ b/include/llvm/CodeGen/MachineRegisterInfo.h @@ -32,6 +32,11 @@ class MachineRegisterInfo { /// registers have a single def. bool IsSSA; + /// TracksLiveness - True while register liveness is being tracked accurately. + /// Basic block live-in lists, kill flags, and implicit defs may not be + /// accurate when after this flag is cleared. + bool TracksLiveness; + /// VRegInfo - Information we keep for each virtual register. /// /// Each element in this list contains the register class of the vreg and the @@ -103,6 +108,23 @@ public: // leaveSSA - Indicates that the machine function is no longer in SSA form. void leaveSSA() { IsSSA = false; } + /// tracksLiveness - Returns true when tracking register liveness accurately. + /// + /// While this flag is true, register liveness information in basic block + /// live-in lists and machine instruction operands is accurate. This means it + /// can be used to change the code in ways that affect the values in + /// registers, for example by the register scavenger. + /// + /// When this flag is false, liveness is no longer reliable. + bool tracksLiveness() const { return TracksLiveness; } + + /// invalidateLiveness - Indicates that register liveness is no longer being + /// tracked accurately. + /// + /// This should be called by late passes that invalidate the liveness + /// information. + void invalidateLiveness() { TracksLiveness = false; } + //===--------------------------------------------------------------------===// // Register Info //===--------------------------------------------------------------------===// diff --git a/lib/CodeGen/MachineRegisterInfo.cpp b/lib/CodeGen/MachineRegisterInfo.cpp index f140decd96..7ea151713a 100644 --- a/lib/CodeGen/MachineRegisterInfo.cpp +++ b/lib/CodeGen/MachineRegisterInfo.cpp @@ -18,7 +18,7 @@ using namespace llvm; MachineRegisterInfo::MachineRegisterInfo(const TargetRegisterInfo &TRI) - : TRI(&TRI), IsSSA(true) { + : TRI(&TRI), IsSSA(true), TracksLiveness(true) { VRegInfo.reserve(256); RegAllocHints.reserve(256); UsedPhysRegs.resize(TRI.getNumRegs()); diff --git a/lib/CodeGen/RegisterScavenging.cpp b/lib/CodeGen/RegisterScavenging.cpp index 2818f490f6..03bd82e225 100644 --- a/lib/CodeGen/RegisterScavenging.cpp +++ b/lib/CodeGen/RegisterScavenging.cpp @@ -83,6 +83,11 @@ void RegScavenger::enterBasicBlock(MachineBasicBlock *mbb) { assert((NumPhysRegs == 0 || NumPhysRegs == TRI->getNumRegs()) && "Target changed?"); + // It is not possible to use the register scavenger after late optimization + // passes that don't preserve accurate liveness information. + assert(MRI->tracksLiveness() && + "Cannot use register scavenger with inaccurate liveness"); + // Self-initialize. if (!MBB) { NumPhysRegs = TRI->getNumRegs(); |