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IndVarSimplify: do not recompute an IV value outside of the loop if :

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- it is trivially known to be used inside the loop in a way that can not be optimized away
- there is no use outside of the loop which can take advantage of the computation hoisting

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@177432 91177308-0d34-0410-b5e6-96231b3b80d8
This commit is contained in:
Arnaud A. de Grandmaison 2013-03-19 20:00:22 +00:00
parent dff4d1522a
commit eb9a42e8ab
2 changed files with 108 additions and 0 deletions
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39
lib/Transforms/Scalar/IndVarSimplify.cpp
View File

@ -535,6 +535,45 @@ void IndVarSimplify::RewriteLoopExitValues(Loop *L, SCEVExpander &Rewriter) {
if (!SE->isLoopInvariant(ExitValue, L))
continue;
// Computing the value outside of the loop brings no benefit if :
// - it is definitely used inside the loop in a way which can not be
// optimized away.
// - no use outside of the loop can take advantage of hoisting the
// computation out of the loop
if (ExitValue->getSCEVType()>=scMulExpr) {
unsigned NumHardInternalUses = 0;
unsigned NumSoftExternalUses = 0;
unsigned NumUses = 0;
for (Value::use_iterator IB=Inst->use_begin(), IE=Inst->use_end();
IB!=IE && NumUses<=6 ; ++IB) {
Instruction *UseInstr = cast<Instruction>(*IB);
unsigned Opc = UseInstr->getOpcode();
NumUses++;
if (L->contains(UseInstr)) {
if (Opc == Instruction::Call || Opc == Instruction::Ret)
NumHardInternalUses++;
} else {
if (Opc == Instruction::PHI) {
// Do not count the Phi as a use. LCSSA may have inserted
// plenty of trivial ones.
NumUses--;
for (Value::use_iterator PB=UseInstr->use_begin(),
PE=UseInstr->use_end();
PB!=PE && NumUses<=6 ; ++PB, ++NumUses) {
unsigned PhiOpc = cast<Instruction>(*PB)->getOpcode();
if (PhiOpc != Instruction::Call && PhiOpc != Instruction::Ret)
NumSoftExternalUses++;
}
continue;
}
if (Opc != Instruction::Call && Opc != Instruction::Ret)
NumSoftExternalUses++;
}
}
if (NumUses <= 6 && NumHardInternalUses && !NumSoftExternalUses)
continue;
}
Value *ExitVal = Rewriter.expandCodeFor(ExitValue, PN->getType(), Inst);
DEBUG(dbgs() << "INDVARS: RLEV: AfterLoopVal = " << *ExitVal << '\n'

69
test/Transforms/IndVarSimplify/dont-recompute.ll Normal file
View File

@ -0,0 +1,69 @@
; RUN: opt < %s -indvars -S | FileCheck %s
; This tests that the IV is not recomputed outside of the loop when it is known
; to be computed by the loop and used in the loop any way. In the example below
; although a's value can be computed outside of the loop, there is no benefit
; in doing so as it has to be computed by the loop anyway.
;
; extern void func(unsigned val);
;
; void test(unsigned m)
; {
; unsigned a = 0;
;
; for (int i=0; i<186; i++) {
; a += m;
; func(a);
; }
;
; func(a);
; }
declare void @func(i32)
; CHECK: @test
define void @test(i32 %m) nounwind uwtable {
entry:
br label %for.body
for.body: ; preds = %for.body, %entry
%i.06 = phi i32 [ 0, %entry ], [ %inc, %for.body ]
%a.05 = phi i32 [ 0, %entry ], [ %add, %for.body ]
%add = add i32 %a.05, %m
; CHECK: tail call void @func(i32 %add)
tail call void @func(i32 %add)
%inc = add nsw i32 %i.06, 1
%exitcond = icmp eq i32 %inc, 186
br i1 %exitcond, label %for.end, label %for.body
for.end: ; preds = %for.body
; CHECK: for.end:
; CHECK-NOT: mul i32 %m, 186
; CHECK:%add.lcssa = phi i32 [ %add, %for.body ]
; CHECK-NEXT: tail call void @func(i32 %add.lcssa)
tail call void @func(i32 %add)
ret void
}
; CHECK: @test2
define i32 @test2(i32 %m) nounwind uwtable {
entry:
br label %for.body
for.body: ; preds = %for.body, %entry
%i.06 = phi i32 [ 0, %entry ], [ %inc, %for.body ]
%a.05 = phi i32 [ 0, %entry ], [ %add, %for.body ]
%add = add i32 %a.05, %m
; CHECK: tail call void @func(i32 %add)
tail call void @func(i32 %add)
%inc = add nsw i32 %i.06, 1
%exitcond = icmp eq i32 %inc, 186
br i1 %exitcond, label %for.end, label %for.body
for.end: ; preds = %for.body
; CHECK: for.end:
; CHECK-NOT: mul i32 %m, 186
; CHECK:%add.lcssa = phi i32 [ %add, %for.body ]
; CHECK-NEXT: ret i32 %add.lcssa
ret i32 %add
}