Enable Local CSE by default
Reduce the default number of iterations to 1
Put the optional code behind the -lcse-no-ssa flag, which is disabled by
default. This brings down the overhead of enabling this to about 2%.
BUG=
R=stichnot@chromium.org
Review URL: https://codereview.chromium.org/2185193002 .
diff --git a/src/IceCfg.cpp b/src/IceCfg.cpp
index 28ae017..b6ab31e 100644
--- a/src/IceCfg.cpp
+++ b/src/IceCfg.cpp
@@ -514,19 +514,23 @@
dump("After basic block shuffling");
}
-void Cfg::localCSE() {
+void Cfg::localCSE(bool AssumeSSA) {
// Performs basic-block local common-subexpression elimination
// If we have
// t1 = op b c
// t2 = op b c
// This pass will replace future references to t2 in a basic block by t1
// Points to note:
- // 1. Does not assume SSA, but not tested on non-SSA input yet as it is run
- // at the beginning.
+ // 1. Assumes SSA by default. To change this, use -lcse=no-ssa
+ // This is needed if this pass is moved to a point later in the pipeline.
+ // If variables have a single definition (in the node), CSE can work just
+ // on the basis of an equality compare on instructions (sans Dest). When
+ // variables can be updated (hence, non-SSA) the result of a previous
+ // instruction which used that variable as an operand can not be reused.
// 2. Leaves removal of instructions to DCE.
// 3. Only enabled on arithmetic instructions. pnacl-clang (-O2) is expected
// to take care of cases not arising from GEP simplification.
- // 4. By default, two passes are made over each basic block. Control this
+ // 4. By default, a single pass is made over each basic block. Control this
// with -lcse-max-iters=N
TimerMarker T(TimerStack::TT_localCse, this);
@@ -575,41 +579,45 @@
for (CfgNode *Node : getNodes()) {
CfgUnorderedSet<Inst *, InstHash, InstEq> Seen;
+ // Stores currently available instructions.
CfgUnorderedMap<Variable *, Variable *, VariableHash> Replacements;
// Combining the above two into a single data structure might consume less
// memory but will be slower i.e map of Instruction -> Set of Variables
CfgUnorderedMap<Variable *, std::vector<Inst *>, VariableHash> Dependency;
- // Not necessary for SSA, still keeping it in case this pass is not run at
- // the beginning. Remove to improve performace.
+ // Maps a variable to the Instructions that depend on it.
+ // a = op1 b c
+ // x = op2 c d
+ // Will result in the map : b -> {a}, c -> {a, x}, d -> {x}
+ // Not necessary for SSA as dependencies will never be invalidated, and the
+ // container will use minimal memory when left unused.
- int IterCount = getFlags().getLocalCseMaxIterations();
+ auto IterCount = getFlags().getLocalCseMaxIterations();
- while (IterCount--) {
- // TODO : Stats on IterCount -> performance
+ for (uint32_t i = 0; i < IterCount; ++i) {
+ // TODO(manasijm): Stats on IterCount -> performance
for (Inst &Instr : Node->getInsts()) {
if (Instr.isDeleted() || !llvm::isa<InstArithmetic>(&Instr))
continue;
+ if (!AssumeSSA) {
+ // Invalidate replacements
+ auto Iter = Replacements.find(Instr.getDest());
+ if (Iter != Replacements.end()) {
+ Replacements.erase(Iter);
+ }
- // Invalidate replacements
- auto Iter = Replacements.find(Instr.getDest());
- if (Iter != Replacements.end()) {
- Replacements.erase(Iter);
- }
-
- // Invalidate 'seen' instructions whose operands were just updated.
- auto DepIter = Dependency.find(Instr.getDest());
- if (DepIter != Dependency.end()) {
- for (auto DepInst : DepIter->second) {
- Seen.erase(DepInst);
+ // Invalidate 'seen' instructions whose operands were just updated.
+ auto DepIter = Dependency.find(Instr.getDest());
+ if (DepIter != Dependency.end()) {
+ for (auto *DepInst : DepIter->second) {
+ Seen.erase(DepInst);
+ }
}
}
- // The above two can be removed if SSA is assumed.
// Replace - doing this before checking for repetitions might enable
- // more
- // optimizations
+ // more optimizations
for (SizeT i = 0; i < Instr.getSrcSize(); ++i) {
auto *Opnd = Instr.getSrc(i);
if (auto *Var = llvm::dyn_cast<Variable>(Opnd)) {
@@ -627,11 +635,13 @@
} else { // new
Seen.insert(&Instr);
- // Update dependencies
- for (SizeT i = 0; i < Instr.getSrcSize(); ++i) {
- auto *Opnd = Instr.getSrc(i);
- if (auto *Var = llvm::dyn_cast<Variable>(Opnd)) {
- Dependency[Var].push_back(&Instr);
+ if (!AssumeSSA) {
+ // Update dependencies
+ for (SizeT i = 0; i < Instr.getSrcSize(); ++i) {
+ auto *Opnd = Instr.getSrc(i);
+ if (auto *Var = llvm::dyn_cast<Variable>(Opnd)) {
+ Dependency[Var].push_back(&Instr);
+ }
}
}
}
diff --git a/src/IceCfg.h b/src/IceCfg.h
index 02e8394..0c37823 100644
--- a/src/IceCfg.h
+++ b/src/IceCfg.h
@@ -201,7 +201,7 @@
void advancedPhiLowering();
void reorderNodes();
void shuffleNodes();
- void localCSE();
+ void localCSE(bool AssumeSSA);
void shortCircuitJumps();
void loopInvariantCodeMotion();
diff --git a/src/IceClFlags.def b/src/IceClFlags.def
index 2af1ee4..0281e7d 100644
--- a/src/IceClFlags.def
+++ b/src/IceClFlags.def
@@ -140,9 +140,14 @@
"information to stdout at the end of program execution."), \
cl::init(false)) \
\
- X(EnableExperimental, bool, dev_opt_flag, "enable-experimental", \
- cl::desc("Enable Optimizations not yet part of O2"), \
- cl::init(false)) \
+ X(LocalCSE, Ice::LCSEOptions, dev_opt_flag, "lcse", \
+ cl::desc("Local common subexpression elimination"), \
+ cl::init(Ice::LCSE_EnabledSSA), \
+ cl::values( \
+ clEnumValN(Ice::LCSE_Disabled, "0", "disabled"), \
+ clEnumValN(Ice::LCSE_EnabledSSA, "enabled", "assume-ssa"), \
+ clEnumValN(Ice::LCSE_EnabledNoSSA, "no-ssa", "no-assume-ssa"), \
+ clEnumValEnd)) \
\
X(EnablePhiEdgeSplit, bool, dev_opt_flag, "phi-edge-split", \
cl::desc("Enable edge splitting for Phi lowering"), cl::init(true)) \
@@ -186,8 +191,8 @@
"building LLVM IR first"), \
cl::init(false)) \
\
- X(LocalCseMaxIterations, int, dev_opt_flag, "lcse-max-iters", \
- cl::desc("Number of times local-cse is run on a block"), cl::init(2)) \
+ X(LocalCseMaxIterations, uint32_t, dev_opt_flag, "lcse-max-iters", \
+ cl::desc("Number of times local-cse is run on a block"), cl::init(1)) \
\
X(LoopInvariantCodeMotion, bool, dev_opt_flag, "licm", \
cl::desc("Hoist loop invariant arithmetic operations"), cl::init(false)) \
diff --git a/src/IceDefs.h b/src/IceDefs.h
index 2c09ba9..c78af0f 100644
--- a/src/IceDefs.h
+++ b/src/IceDefs.h
@@ -308,6 +308,12 @@
Liveness_Intervals
};
+enum LCSEOptions {
+ LCSE_Disabled,
+ LCSE_EnabledSSA, // Default Mode, assumes SSA.
+ LCSE_EnabledNoSSA // Does not assume SSA, to be enabled if CSE is done later.
+};
+
enum RegAllocKind {
RAK_Unknown,
RAK_Global, /// full, global register allocation
diff --git a/src/IceTargetLoweringX86BaseImpl.h b/src/IceTargetLoweringX86BaseImpl.h
index ecb4ae6..27fa85f 100644
--- a/src/IceTargetLoweringX86BaseImpl.h
+++ b/src/IceTargetLoweringX86BaseImpl.h
@@ -457,8 +457,8 @@
Func->dump("After LICM");
}
- if (getFlags().getEnableExperimental()) {
- Func->localCSE();
+ if (getFlags().getLocalCSE() != Ice::LCSE_Disabled) {
+ Func->localCSE(getFlags().getLocalCSE() == Ice::LCSE_EnabledSSA);
Func->dump("After Local CSE");
}
if (getFlags().getEnableShortCircuit()) {
diff --git a/tests_lit/llvm2ice_tests/local-cse.ll b/tests_lit/llvm2ice_tests/local-cse.ll
index fae1816..b99c8f4 100644
--- a/tests_lit/llvm2ice_tests/local-cse.ll
+++ b/tests_lit/llvm2ice_tests/local-cse.ll
@@ -1,18 +1,18 @@
; Tests local-cse on x8632 and x8664
; RUN: %p2i -i %s --filetype=obj --disassemble --target x8632 --args \
-; RUN: -O2 -enable-experimental | FileCheck --check-prefix=X8632 \
+; RUN: -O2 | FileCheck --check-prefix=X8632 \
; RUN: --check-prefix=X8632EXP %s
; RUN: %p2i -i %s --filetype=obj --disassemble --target x8632 --args \
-; RUN: -O2 | FileCheck --check-prefix=X8632 --check-prefix X8632NOEXP %s
+; RUN: -O2 -lcse=0| FileCheck --check-prefix=X8632 --check-prefix X8632NOEXP %s
; RUN: %p2i -i %s --filetype=obj --disassemble --target x8664 --args \
-; RUN: -O2 -enable-experimental | FileCheck --check-prefix=X8664 \
+; RUN: -O2 | FileCheck --check-prefix=X8664 \
; RUN: --check-prefix=X8664EXP %s
; RUN: %p2i -i %s --filetype=obj --disassemble --target x8664 --args \
-; RUN: -O2 | FileCheck --check-prefix=X8664 --check-prefix X8664NOEXP %s
+; RUN: -O2 -lcse=0| FileCheck --check-prefix=X8664 --check-prefix X8664NOEXP %s
define internal i32 @local_cse_test(i32 %a, i32 %b) {
