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Don't brute-force analyze cubic or higher polynomials.

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If this patch causes a performance regression for anyone, please let me know,
and it can be fixed in a different way with much more effort.


git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@59384 91177308-0d34-0410-b5e6-96231b3b80d8
This commit is contained in:
Nick Lewycky 2008-11-16 04:14:25 +00:00
parent 808e3a9266
commit d72a81ee64
2 changed files with 19 additions and 21 deletions
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21
lib/Analysis/ScalarEvolution.cpp
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@ -2967,27 +2967,6 @@ SCEVHandle SCEVAddRecExpr::getNumIterationsInRange(ConstantRange Range,
}
}
// Fallback, if this is a general polynomial, figure out the progression
// through brute force: evaluate until we find an iteration that fails the
// test. This is likely to be slow, but getting an accurate trip count is
// incredibly important, we will be able to simplify the exit test a lot, and
// we are almost guaranteed to get a trip count in this case.
ConstantInt *TestVal = ConstantInt::get(getType(), 0);
ConstantInt *EndVal = TestVal; // Stop when we wrap around.
do {
++NumBruteForceEvaluations;
SCEVHandle Val = evaluateAtIteration(SE.getConstant(TestVal), SE);
if (!isa<SCEVConstant>(Val)) // This shouldn't happen.
return new SCEVCouldNotCompute();
// Check to see if we found the value!
if (!Range.contains(cast<SCEVConstant>(Val)->getValue()->getValue()))
return SE.getConstant(TestVal);
// Increment to test the next index.
TestVal = ConstantInt::get(TestVal->getValue()+1);
} while (TestVal != EndVal);
return new SCEVCouldNotCompute();
}

19
test/Analysis/ScalarEvolution/2008-11-15-CubicOOM.ll Normal file
View File

@ -0,0 +1,19 @@
; RUN: llvm-as < %s | opt -analyze -scalar-evolution
; PR2602
define i32 @a() nounwind {
entry:
br label %bb
bb: ; preds = %bb, %entry
%w.0 = phi i32 [ 0, %entry ], [ %tmp, %bb ] ; <i32> [#uses=2]
%e.0 = phi i32 [ 0, %entry ], [ %e.1, %bb ] ; <i32> [#uses=2]
%w.1 = add i32 0, %w.0 ; <i32>:0 [#uses=1]
%tmp = add i32 %e.0, %w.0 ; <i32>:1 [#uses=1]
%e.1 = add i32 %e.0, 1 ; <i32>:2 [#uses=1]
%cond = icmp eq i32 %w.1, -1 ; <i1>:3 [#uses=1]
br i1 %cond, label %return, label %bb
return: ; preds = %bb
ret i32 undef
}