[llvm-commits] [llvm] r152405 - in /llvm/trunk: lib/Transforms/Scalar/CorrelatedValuePropagation.cpp test/Transforms/CorrelatedValuePropagation/basic.ll
Duncan Sands
baldrick at free.fr
Fri Mar 9 05:45:18 PST 2012
Author: baldrick
Date: Fri Mar 9 07:45:18 2012
New Revision: 152405
URL: http://llvm.org/viewvc/llvm-project?rev=152405&view=rev
Log:
Eliminate switch cases that can never match, for example removes all
negative switch cases if the branch condition is known to be positive.
Inspired by a recent improvement to GCC's VRP.
Modified:
llvm/trunk/lib/Transforms/Scalar/CorrelatedValuePropagation.cpp
llvm/trunk/test/Transforms/CorrelatedValuePropagation/basic.ll
Modified: llvm/trunk/lib/Transforms/Scalar/CorrelatedValuePropagation.cpp
URL: http://llvm.org/viewvc/llvm-project/llvm/trunk/lib/Transforms/Scalar/CorrelatedValuePropagation.cpp?rev=152405&r1=152404&r2=152405&view=diff
==============================================================================
--- llvm/trunk/lib/Transforms/Scalar/CorrelatedValuePropagation.cpp (original)
+++ llvm/trunk/lib/Transforms/Scalar/CorrelatedValuePropagation.cpp Fri Mar 9 07:45:18 2012
@@ -37,6 +37,7 @@
bool processPHI(PHINode *P);
bool processMemAccess(Instruction *I);
bool processCmp(CmpInst *C);
+ bool processSwitch(SwitchInst *SI);
public:
static char ID;
@@ -173,6 +174,84 @@
return true;
}
+/// processSwitch - Simplify a switch instruction by removing cases which can
+/// never fire. If the uselessness of a case could be determined locally then
+/// constant propagation would already have figured it out. Instead, walk the
+/// predecessors and statically evaluate cases based on information available
+/// on that edge. Cases that cannot fire no matter what the incoming edge can
+/// safely be removed. If a case fires on every incoming edge then the entire
+/// switch can be removed and replaced with a branch to the case destination.
+bool CorrelatedValuePropagation::processSwitch(SwitchInst *SI) {
+ Value *Cond = SI->getCondition();
+ BasicBlock *BB = SI->getParent();
+
+ // If the condition was defined in same block as the switch then LazyValueInfo
+ // currently won't say anything useful about it, though in theory it could.
+ if (isa<Instruction>(Cond) && cast<Instruction>(Cond)->getParent() == BB)
+ return false;
+
+ // If the switch is unreachable then trying to improve it is a waste of time.
+ pred_iterator PB = pred_begin(BB), PE = pred_end(BB);
+ if (PB == PE) return false;
+
+ // Analyse each switch case in turn. This is done in reverse order so that
+ // removing a case doesn't cause trouble for the iteration.
+ bool Changed = false;
+ for (SwitchInst::CaseIt CI = SI->caseEnd(), CE = SI->caseBegin(); CI-- != CE;
+ ) {
+ ConstantInt *Case = CI.getCaseValue();
+
+ // Check to see if the switch condition is equal to/not equal to the case
+ // value on every incoming edge, equal/not equal being the same each time.
+ LazyValueInfo::Tristate State = LazyValueInfo::Unknown;
+ for (pred_iterator PI = PB; PI != PE; ++PI) {
+ // Is the switch condition equal to the case value?
+ LazyValueInfo::Tristate Value = LVI->getPredicateOnEdge(CmpInst::ICMP_EQ,
+ Cond, Case, *PI, BB);
+ // Give up on this case if nothing is known.
+ if (Value == LazyValueInfo::Unknown) {
+ State = LazyValueInfo::Unknown;
+ break;
+ }
+
+ // If this was the first edge to be visited, record that all other edges
+ // need to give the same result.
+ if (PI == PB) {
+ State = Value;
+ continue;
+ }
+
+ // If this case is known to fire for some edges and known not to fire for
+ // others then there is nothing we can do - give up.
+ if (Value != State) {
+ State = LazyValueInfo::Unknown;
+ break;
+ }
+ }
+
+ if (State == LazyValueInfo::False) {
+ // This case never fires - remove it.
+ CI.getCaseSuccessor()->removePredecessor(BB);
+ SI->removeCase(CI); // Does not invalidate the iterator.
+ Changed = true;
+ } else if (State == LazyValueInfo::True) {
+ // This case always fires. Arrange for the switch to be turned into an
+ // unconditional branch by replacing the switch condition with the case
+ // value.
+ SI->setCondition(Case);
+ Changed = true;
+ break;
+ }
+ }
+
+ if (Changed)
+ // If the switch has been simplified to the point where it can be replaced
+ // by a branch then do so now.
+ ConstantFoldTerminator(BB);
+
+ return Changed;
+}
+
bool CorrelatedValuePropagation::runOnFunction(Function &F) {
LVI = &getAnalysis<LazyValueInfo>();
@@ -200,6 +279,13 @@
}
}
+ Instruction *Term = FI->getTerminator();
+ switch (Term->getOpcode()) {
+ case Instruction::Switch:
+ BBChanged |= processSwitch(cast<SwitchInst>(Term));
+ break;
+ }
+
FnChanged |= BBChanged;
}
Modified: llvm/trunk/test/Transforms/CorrelatedValuePropagation/basic.ll
URL: http://llvm.org/viewvc/llvm-project/llvm/trunk/test/Transforms/CorrelatedValuePropagation/basic.ll?rev=152405&r1=152404&r2=152405&view=diff
==============================================================================
--- llvm/trunk/test/Transforms/CorrelatedValuePropagation/basic.ll (original)
+++ llvm/trunk/test/Transforms/CorrelatedValuePropagation/basic.ll Fri Mar 9 07:45:18 2012
@@ -79,4 +79,103 @@
LessThanOrEqualToTwo:
ret i32 0
-}
\ No newline at end of file
+}
+
+define i32 @switch1(i32 %s) {
+; CHECK: @switch1
+entry:
+ %cmp = icmp slt i32 %s, 0
+ br i1 %cmp, label %negative, label %out
+
+negative:
+ switch i32 %s, label %out [
+; CHECK: switch i32 %s, label %out
+ i32 0, label %out
+; CHECK-NOT: i32 0
+ i32 1, label %out
+; CHECK-NOT: i32 1
+ i32 -1, label %next
+; CHECK: i32 -1, label %next
+ i32 -2, label %next
+; CHECK: i32 -2, label %next
+ i32 2, label %out
+; CHECK-NOT: i32 2
+ i32 3, label %out
+; CHECK-NOT: i32 3
+ ]
+
+out:
+ %p = phi i32 [ 1, %entry ], [ -1, %negative ], [ -1, %negative ], [ -1, %negative ], [ -1, %negative ]
+ ret i32 %p
+
+next:
+ %q = phi i32 [ 0, %negative ], [ 0, %negative ]
+ ret i32 %q
+}
+
+define i32 @switch2(i32 %s) {
+; CHECK: @switch2
+entry:
+ %cmp = icmp sgt i32 %s, 0
+ br i1 %cmp, label %positive, label %out
+
+positive:
+ switch i32 %s, label %out [
+ i32 0, label %out
+ i32 -1, label %next
+ i32 -2, label %next
+ ]
+; CHECK: br label %out
+
+out:
+ %p = phi i32 [ -1, %entry ], [ 1, %positive ], [ 1, %positive ]
+ ret i32 %p
+
+next:
+ %q = phi i32 [ 0, %positive ], [ 0, %positive ]
+ ret i32 %q
+}
+
+define i32 @switch3(i32 %s) {
+; CHECK: @switch3
+entry:
+ %cmp = icmp sgt i32 %s, 0
+ br i1 %cmp, label %positive, label %out
+
+positive:
+ switch i32 %s, label %out [
+ i32 -1, label %out
+ i32 -2, label %next
+ i32 -3, label %next
+ ]
+; CHECK: br label %out
+
+out:
+ %p = phi i32 [ -1, %entry ], [ 1, %positive ], [ 1, %positive ]
+ ret i32 %p
+
+next:
+ %q = phi i32 [ 0, %positive ], [ 0, %positive ]
+ ret i32 %q
+}
+
+define void @switch4(i32 %s) {
+; CHECK: @switch4
+entry:
+ %cmp = icmp eq i32 %s, 0
+ br i1 %cmp, label %zero, label %out
+
+zero:
+ switch i32 %s, label %out [
+ i32 0, label %next
+ i32 1, label %out
+ i32 -1, label %out
+ ]
+; CHECK: br label %next
+
+out:
+ ret void
+
+next:
+ ret void
+}
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