atmnpatel created this revision.
atmnpatel added reviewers: jdoerfert, aqjune, RalfJung.
Herald added subscribers: cfe-commits, danielkiss.
Herald added a project: clang.
atmnpatel requested review of this revision.
The `noprogress` LLVM IR attribute currently under discussion is deduced for
functions that contain loops that have a constant non-zero conditional as per
the C Spec. In addition, these particular loops have an additional bit of
metadata `llvm.loop.noprogress` so llvm can differentiate between the loops
that can be optimized out and ones that can't be optimized out. The changes to
the LoopDeletion pass is incoming.
Repository:
rG LLVM Github Monorepo
https://reviews.llvm.org/D86841
Files:
clang/lib/CodeGen/CGLoopInfo.cpp
clang/lib/CodeGen/CGLoopInfo.h
clang/lib/CodeGen/CGStmt.cpp
clang/test/CodeGen/attr-noprogress.c
Index: clang/test/CodeGen/attr-noprogress.c
===================================================================
--- /dev/null
+++ clang/test/CodeGen/attr-noprogress.c
@@ -0,0 +1,18 @@
+// RUN: %clang_cc1 -S -emit-llvm %s -o - | grep " noprogress " | count 1
+// RUN: %clang_cc1 -S -emit-llvm %s -o - | grep llvm.loop.noprogress | count 1
+
+void f() {
+ for (; 1;) {
+ }
+}
+
+void w() {
+ while (1) {
+ }
+}
+
+void d() {
+ do {
+
+ } while (1);
+}
Index: clang/lib/CodeGen/CGStmt.cpp
===================================================================
--- clang/lib/CodeGen/CGStmt.cpp
+++ clang/lib/CodeGen/CGStmt.cpp
@@ -736,10 +736,26 @@
JumpDest LoopHeader = getJumpDestInCurrentScope("while.cond");
EmitBlock(LoopHeader.getBlock());
+ // Evaluate the conditional in the while header. C99 6.8.5.1: The
+ // evaluation of the controlling expression takes place before each
+ // execution of the loop body.
+ llvm::Value *BoolCondVal = EvaluateExprAsBool(S.getCond());
+
+ // while(1) is common, avoid extra exit blocks. Be sure
+ // to correctly handle break/continue though.
+ bool EmitBoolCondBranch = true;
+ bool NoProgress = false;
+ if (llvm::ConstantInt *C = dyn_cast<llvm::ConstantInt>(BoolCondVal))
+ if (C->isOne()) {
+ EmitBoolCondBranch = false;
+ CurFn->addFnAttr(llvm::Attribute::NoProgress);
+ NoProgress = true;
+ }
+
const SourceRange &R = S.getSourceRange();
LoopStack.push(LoopHeader.getBlock(), CGM.getContext(), CGM.getCodeGenOpts(),
WhileAttrs, SourceLocToDebugLoc(R.getBegin()),
- SourceLocToDebugLoc(R.getEnd()));
+ SourceLocToDebugLoc(R.getEnd()), NoProgress);
// Create an exit block for when the condition fails, which will
// also become the break target.
@@ -760,18 +776,6 @@
if (S.getConditionVariable())
EmitDecl(*S.getConditionVariable());
- // Evaluate the conditional in the while header. C99 6.8.5.1: The
- // evaluation of the controlling expression takes place before each
- // execution of the loop body.
- llvm::Value *BoolCondVal = EvaluateExprAsBool(S.getCond());
-
- // while(1) is common, avoid extra exit blocks. Be sure
- // to correctly handle break/continue though.
- bool EmitBoolCondBranch = true;
- if (llvm::ConstantInt *C = dyn_cast<llvm::ConstantInt>(BoolCondVal))
- if (C->isOne())
- EmitBoolCondBranch = false;
-
// As long as the condition is true, go to the loop body.
llvm::BasicBlock *LoopBody = createBasicBlock("while.body");
if (EmitBoolCondBranch) {
@@ -838,27 +842,33 @@
EmitBlock(LoopCond.getBlock());
- const SourceRange &R = S.getSourceRange();
- LoopStack.push(LoopBody, CGM.getContext(), CGM.getCodeGenOpts(), DoAttrs,
- SourceLocToDebugLoc(R.getBegin()),
- SourceLocToDebugLoc(R.getEnd()));
-
- // C99 6.8.5.2: "The evaluation of the controlling expression takes place
- // after each execution of the loop body."
-
// Evaluate the conditional in the while header.
// C99 6.8.5p2/p4: The first substatement is executed if the expression
// compares unequal to 0. The condition must be a scalar type.
llvm::Value *BoolCondVal = EvaluateExprAsBool(S.getCond());
- BreakContinueStack.pop_back();
-
// "do {} while (0)" is common in macros, avoid extra blocks. Be sure
// to correctly handle break/continue though.
bool EmitBoolCondBranch = true;
- if (llvm::ConstantInt *C = dyn_cast<llvm::ConstantInt>(BoolCondVal))
+ bool NoProgress = false;
+ if (llvm::ConstantInt *C = dyn_cast<llvm::ConstantInt>(BoolCondVal)) {
if (C->isZero())
EmitBoolCondBranch = false;
+ if (C->isOne()) {
+ CurFn->addFnAttr(llvm::Attribute::NoProgress);
+ NoProgress = true;
+ }
+ }
+
+ const SourceRange &R = S.getSourceRange();
+ LoopStack.push(LoopBody, CGM.getContext(), CGM.getCodeGenOpts(), DoAttrs,
+ SourceLocToDebugLoc(R.getBegin()),
+ SourceLocToDebugLoc(R.getEnd()), NoProgress);
+
+ // C99 6.8.5.2: "The evaluation of the controlling expression takes place
+ // after each execution of the loop body."
+
+ BreakContinueStack.pop_back();
// As long as the condition is true, iterate the loop.
if (EmitBoolCondBranch) {
@@ -896,10 +906,20 @@
llvm::BasicBlock *CondBlock = Continue.getBlock();
EmitBlock(CondBlock);
+ bool NoProgress = false;
+ if (S.getCond()) {
+ llvm::Value *BoolCondVal = EvaluateExprAsBool(S.getCond());
+ if (llvm::ConstantInt *C = dyn_cast<llvm::ConstantInt>(BoolCondVal))
+ if (C->isOne()) {
+ CurFn->addFnAttr(llvm::Attribute::NoProgress);
+ NoProgress = true;
+ }
+ }
+
const SourceRange &R = S.getSourceRange();
LoopStack.push(CondBlock, CGM.getContext(), CGM.getCodeGenOpts(), ForAttrs,
SourceLocToDebugLoc(R.getBegin()),
- SourceLocToDebugLoc(R.getEnd()));
+ SourceLocToDebugLoc(R.getEnd()), NoProgress);
// If the for loop doesn't have an increment we can just use the
// condition as the continue block. Otherwise we'll need to create
Index: clang/lib/CodeGen/CGLoopInfo.h
===================================================================
--- clang/lib/CodeGen/CGLoopInfo.h
+++ clang/lib/CodeGen/CGLoopInfo.h
@@ -75,6 +75,9 @@
/// Value for llvm.loop.pipeline.iicount metadata.
unsigned PipelineInitiationInterval;
+
+ /// Value to prevent Dead Loop Elimination.
+ bool NoProgress;
};
/// Information used when generating a structured loop.
@@ -205,7 +208,7 @@
void push(llvm::BasicBlock *Header, clang::ASTContext &Ctx,
const clang::CodeGenOptions &CGOpts,
llvm::ArrayRef<const Attr *> Attrs, const llvm::DebugLoc &StartLoc,
- const llvm::DebugLoc &EndLoc);
+ const llvm::DebugLoc &EndLoc, const bool NoProgress = false);
/// End the current loop.
void pop();
@@ -272,6 +275,9 @@
StagedAttrs.PipelineInitiationInterval = C;
}
+ /// Set no progress for the next loop pushed.
+ void setNoProgress(bool P) { StagedAttrs.NoProgress = P; }
+
private:
/// Returns true if there is LoopInfo on the stack.
bool hasInfo() const { return !Active.empty(); }
Index: clang/lib/CodeGen/CGLoopInfo.cpp
===================================================================
--- clang/lib/CodeGen/CGLoopInfo.cpp
+++ clang/lib/CodeGen/CGLoopInfo.cpp
@@ -418,10 +418,14 @@
LoopProperties.push_back(EndLoc.getAsMDNode());
}
+ LLVMContext &Ctx = Header->getContext();
+ if (Attrs.NoProgress)
+ LoopProperties.push_back(
+ MDNode::get(Ctx, MDString::get(Ctx, "llvm.loop.noprogress")));
+
assert(!!AccGroup == Attrs.IsParallel &&
"There must be an access group iff the loop is parallel");
if (Attrs.IsParallel) {
- LLVMContext &Ctx = Header->getContext();
LoopProperties.push_back(MDNode::get(
Ctx, {MDString::get(Ctx, "llvm.loop.parallel_accesses"), AccGroup}));
}
@@ -438,7 +442,7 @@
VectorizePredicateEnable(LoopAttributes::Unspecified), VectorizeWidth(0),
InterleaveCount(0), UnrollCount(0), UnrollAndJamCount(0),
DistributeEnable(LoopAttributes::Unspecified), PipelineDisabled(false),
- PipelineInitiationInterval(0) {}
+ PipelineInitiationInterval(0), NoProgress(false) {}
void LoopAttributes::clear() {
IsParallel = false;
@@ -453,6 +457,7 @@
DistributeEnable = LoopAttributes::Unspecified;
PipelineDisabled = false;
PipelineInitiationInterval = 0;
+ NoProgress = false;
}
LoopInfo::LoopInfo(BasicBlock *Header, const LoopAttributes &Attrs,
@@ -476,7 +481,7 @@
Attrs.UnrollEnable == LoopAttributes::Unspecified &&
Attrs.UnrollAndJamEnable == LoopAttributes::Unspecified &&
Attrs.DistributeEnable == LoopAttributes::Unspecified && !StartLoc &&
- !EndLoc)
+ !EndLoc && !Attrs.NoProgress)
return;
TempLoopID = MDNode::getTemporary(Header->getContext(), None);
@@ -577,8 +582,7 @@
const clang::CodeGenOptions &CGOpts,
ArrayRef<const clang::Attr *> Attrs,
const llvm::DebugLoc &StartLoc,
- const llvm::DebugLoc &EndLoc) {
-
+ const llvm::DebugLoc &EndLoc, const bool NoProgress) {
// Identify loop hint attributes from Attrs.
for (const auto *Attr : Attrs) {
const LoopHintAttr *LH = dyn_cast<LoopHintAttr>(Attr);
@@ -755,6 +759,8 @@
}
}
+ setNoProgress(NoProgress);
+
if (CGOpts.OptimizationLevel > 0)
// Disable unrolling for the loop, if unrolling is disabled (via
// -fno-unroll-loops) and no pragmas override the decision.
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