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First tiny step to implementing PR9322: build infrastructure for only emitting the 

live case of a switch statement when switching on a constant.  This is terribly
limited, but enough to handle the trivial example included.  Before we would 
emit:

define void @test1(i32 %i) nounwind {
entry:
  %i.addr = alloca i32, align 4
  store i32 %i, i32* %i.addr, align 4
  switch i32 1, label %sw.epilog [
    i32 1, label %sw.bb
  ]

sw.bb:                                            ; preds = %entry
  %tmp = load i32* %i.addr, align 4
  %inc = add nsw i32 %tmp, 1
  store i32 %inc, i32* %i.addr, align 4
  br label %sw.epilog

sw.epilog:                                        ; preds = %sw.bb, %entry
  switch i32 0, label %sw.epilog3 [
    i32 1, label %sw.bb1
  ]

sw.bb1:                                           ; preds = %sw.epilog
  %tmp2 = load i32* %i.addr, align 4
  %add = add nsw i32 %tmp2, 2
  store i32 %add, i32* %i.addr, align 4
  br label %sw.epilog3

sw.epilog3:                                       ; preds = %sw.bb1, %sw.epilog
  ret void
}

now we emit:

define void @test1(i32 %i) nounwind {
entry:
  %i.addr = alloca i32, align 4
  store i32 %i, i32* %i.addr, align 4
  %tmp = load i32* %i.addr, align 4
  %inc = add nsw i32 %tmp, 1
  store i32 %inc, i32* %i.addr, align 4
  ret void
}

This improves -O0 compile time (less IR to generate and shove through the code 
generator) and the clever linux kernel people found a way to fail to build if we 
don't do this optimization.  This step isn't enough to handle the kernel case
though.

llvm-svn: 126597
This commit is contained in:
Chris Lattner 2011-02-28 00:22:07 +00:00
parent 29911cc2f5
commit 0725a8b653
2 changed files with 151 additions and 0 deletions
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133
clang/lib/CodeGen/CGStmt.cpp
View File

@ -829,6 +829,122 @@ void CodeGenFunction::EmitDefaultStmt(const DefaultStmt &S) {
EmitStmt(S.getSubStmt());
}
/// CollectStatementsForCase - Given the body of a 'switch' statement and a
/// constant value that is being switched on, see if we can dead code eliminate
/// the body of the switch to a simple series of statements to emit. Basically,
/// on a switch (5) we want to find these statements:
/// case 5:
/// printf(...); <--
/// ++i; <--
/// break;
///
/// and add them to the ResultStmts vector. If it is unsafe to do this
/// transformation (for example, one of the elided statements contains a label
/// that might be jumped to), return CSFC_Failure. If we handled it and 'S'
/// should include statements after it (e.g. the printf() line is a substmt of
/// the case) then return CSFC_FallThrough. If we handled it and found a break
/// statement, then return CSFC_Success.
///
/// If Case is non-null, then we are looking for the specified case, checking
/// that nothing we jump over contains labels. If Case is null, then we found
/// the case and are looking for the break.
///
/// If the recursive walk actually finds our Case, then we set FoundCase to
/// true.
///
enum CSFC_Result { CSFC_Failure, CSFC_FallThrough, CSFC_Success };
static CSFC_Result CollectStatementsForCase(const Stmt *S,
const SwitchCase *Case,
bool &FoundCase,
llvm::SmallVectorImpl<const Stmt*> &ResultStmts) {
// If this is the switchcase (case 4: or default) that we're looking for, then
// we're in business. Just add the substatement.
if (const SwitchCase *SC = dyn_cast<SwitchCase>(S)) {
if (S == Case) {
FoundCase = true;
return CollectStatementsForCase(SC->getSubStmt(), 0, FoundCase,
ResultStmts);
}
// Otherwise, this is some other case or default statement, just ignore it.
return CollectStatementsForCase(SC->getSubStmt(), Case, FoundCase,
ResultStmts);
}
// Okay, this is some other statement that we don't handle explicitly, like a
// for statement or increment etc. If we are skipping over this statement,
// just verify it doesn't have labels, which would make it invalid to elide.
if (Case) {
if (CodeGenFunction::ContainsLabel(S, true))
return CSFC_Failure;
return CSFC_Success;
}
// Otherwise, we want to include this statement. Everything is cool with that
// so long as it doesn't contain a break out of the switch we're in.
if (CodeGenFunction::containsBreak(S)) return CSFC_Failure;
// Otherwise, everything is great. Include the statement and tell the caller
// that we fall through and include the next statement as well.
ResultStmts.push_back(S);
return CSFC_FallThrough;
}
/// FindCaseStatementsForValue - Find the case statement being jumped to and
/// then invoke CollectStatementsForCase to find the list of statements to emit
/// for a switch on constant. See the comment above CollectStatementsForCase
/// for more details.
static bool FindCaseStatementsForValue(const SwitchStmt &S,
const llvm::APInt &ConstantCondValue,
llvm::SmallVectorImpl<const Stmt*> &ResultStmts,
ASTContext &C) {
// First step, find the switch case that is being branched to. We can do this
// efficiently by scanning the SwitchCase list.
const SwitchCase *Case = S.getSwitchCaseList();
const DefaultStmt *DefaultCase = 0;
for (; Case; Case = Case->getNextSwitchCase()) {
// It's either a default or case. Just remember the default statement in
// case we're not jumping to any numbered cases.
if (const DefaultStmt *DS = dyn_cast<DefaultStmt>(Case)) {
DefaultCase = DS;
continue;
}
// Check to see if this case is the one we're looking for.
const CaseStmt *CS = cast<CaseStmt>(Case);
// Don't handle case ranges yet.
if (CS->getRHS()) return false;
// If we found our case, remember it as 'case'.
if (CS->getLHS()->EvaluateAsInt(C) == ConstantCondValue)
break;
}
// If we didn't find a matching case, we use a default if it exists, or we
// elide the whole switch body!
if (Case == 0) {
// It is safe to elide the body of the switch if it doesn't contain labels
// etc. If it is safe, return successfully with an empty ResultStmts list.
if (DefaultCase == 0)
return !CodeGenFunction::ContainsLabel(&S);
Case = DefaultCase;
}
// Ok, we know which case is being jumped to, try to collect all the
// statements that follow it. This can fail for a variety of reasons. Also,
// check to see that the recursive walk actually found our case statement.
// Insane cases like this can fail to find it in the recursive walk since we
// don't handle every stmt kind:
// switch (4) {
// while (1) {
// case 4: ...
bool FoundCase = false;
return CollectStatementsForCase(S.getBody(), Case, FoundCase,
ResultStmts) != CSFC_Failure &&
FoundCase;
}
void CodeGenFunction::EmitSwitchStmt(const SwitchStmt &S) {
JumpDest SwitchExit = getJumpDestInCurrentScope("sw.epilog");
@ -837,6 +953,23 @@ void CodeGenFunction::EmitSwitchStmt(const SwitchStmt &S) {
if (S.getConditionVariable())
EmitAutoVarDecl(*S.getConditionVariable());
// See if we can constant fold the condition of the switch and therefore only
// emit the live case statement (if any) of the switch.
llvm::APInt ConstantCondValue;
if (ConstantFoldsToSimpleInteger(S.getCond(), ConstantCondValue)) {
llvm::SmallVector<const Stmt*, 4> CaseStmts;
if (FindCaseStatementsForValue(S, ConstantCondValue, CaseStmts,
getContext())) {
RunCleanupsScope ExecutedScope(*this);
// Okay, we can dead code eliminate everything except this case. Emit the
// specified series of statements and we're good.
for (unsigned i = 0, e = CaseStmts.size(); i != e; ++i)
EmitStmt(CaseStmts[i]);
return;
}
}
llvm::Value *CondV = EmitScalarExpr(S.getCond());
// Handle nested switch statements.

18
clang/test/CodeGen/switch-dce.c Normal file
View File

@ -0,0 +1,18 @@
// RUN: %clang_cc1 -triple i386-unknown-unknown -O0 %s -emit-llvm -o - | FileCheck %s
// CHECK: @test1
// CHECK-NOT: switch
// CHECK: add nsw i32 {{.*}}, 1
// CHECK-NOT: switch
// CHECK-NOT: add nsw i32
// CHECK: ret void
void test1() {
int i;
switch (1)
case 1:
++i;
switch (0)
case 1:
i+=2;
}