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Improve tail call elimination to handle the switch statement. 

llvm-svn: 86403
This commit is contained in:
Nick Lewycky 2009-11-07 21:10:15 +00:00
parent d7aa9d8a63
commit b9397262b7
3 changed files with 45 additions and 18 deletions
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16
llvm/lib/Target/README.txt
View File

@ -406,22 +406,6 @@ return: ; preds = %then.1, %else.0, %then.0
//===---------------------------------------------------------------------===//
Tail recursion elimination is not transforming this function, because it is
returning n, which fails the isDynamicConstant check in the accumulator
recursion checks.
long long fib(const long long n) {
switch(n) {
case 0:
case 1:
return n;
default:
return fib(n-1) + fib(n-2);
}
}
//===---------------------------------------------------------------------===//
Tail recursion elimination should handle:
int pow2m1(int n) {

13
llvm/lib/Transforms/Scalar/TailRecursionElimination.cpp
View File

@ -234,7 +234,7 @@ bool TailCallElim::CanMoveAboveCall(Instruction *I, CallInst *CI) {
// We currently handle static constants and arguments that are not modified as
// part of the recursion.
//
static bool isDynamicConstant(Value *V, CallInst *CI) {
static bool isDynamicConstant(Value *V, CallInst *CI, ReturnInst *RI) {
if (isa<Constant>(V)) return true; // Static constants are always dyn consts
// Check to see if this is an immutable argument, if so, the value
@ -252,6 +252,15 @@ static bool isDynamicConstant(Value *V, CallInst *CI) {
if (CI->getOperand(ArgNo+1) == Arg)
return true;
}
// Switch cases are always constant integers. If the value is being switched
// on and the return is only reachable from one of its cases, it's
// effectively constant.
if (BasicBlock *UniquePred = RI->getParent()->getUniquePredecessor())
if (SwitchInst *SI = dyn_cast<SwitchInst>(UniquePred->getTerminator()))
if (SI->getCondition() == V)
return SI->getDefaultDest() != RI->getParent();
// Not a constant or immutable argument, we can't safely transform.
return false;
}
@ -273,7 +282,7 @@ static Value *getCommonReturnValue(ReturnInst *TheRI, CallInst *CI) {
// evaluatable at the start of the initial invocation of the function,
// instead of at the end of the evaluation.
//
if (!isDynamicConstant(RetOp, CI))
if (!isDynamicConstant(RetOp, CI, RI))
return 0;
if (ReturnedValue && RetOp != ReturnedValue)

34
llvm/test/Transforms/TailCallElim/switch.ll Normal file
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@ -0,0 +1,34 @@
; RUN: opt %s -tailcallelim -S | FileCheck %s
define i64 @fib(i64 %n) nounwind readnone {
; CHECK: @fib
entry:
; CHECK: tailrecurse:
; CHECK: %accumulator.tr = phi i64 [ %n, %entry ], [ %3, %bb1 ]
; CHECK: %n.tr = phi i64 [ %n, %entry ], [ %2, %bb1 ]
switch i64 %n, label %bb1 [
; CHECK: switch i64 %n.tr, label %bb1 [
i64 0, label %bb2
i64 1, label %bb2
]
bb1:
; CHECK: bb1:
%0 = add i64 %n, -1
; CHECK: %0 = add i64 %n.tr, -1
%1 = tail call i64 @fib(i64 %0) nounwind
; CHECK: %1 = tail call i64 @fib(i64 %0)
%2 = add i64 %n, -2
; CHECK: %2 = add i64 %n.tr, -2
%3 = tail call i64 @fib(i64 %2) nounwind
; CHECK-NOT: tail call i64 @fib
%4 = add nsw i64 %3, %1
; CHECK: add nsw i64 %accumulator.tr, %1
ret i64 %4
; CHECK: br label %tailrecurse
bb2:
; CHECK: bb2:
ret i64 %n
; CHECK: ret i64 %accumulator.tr
}