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Fix SCEV overly optimistic back edge taken count for multi-exit loops.

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Fixes PR11375: Different results for 'clang++ huh.cpp'...


git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@144746 91177308-0d34-0410-b5e6-96231b3b80d8
This commit is contained in:
Andrew Trick 2011-11-16 00:52:40 +00:00
parent f56c60b571
commit 79f0bfcc20
4 changed files with 75 additions and 11 deletions
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29
lib/Analysis/ScalarEvolution.cpp
View File

@ -4153,13 +4153,19 @@ void ScalarEvolution::forgetValue(Value *V) {
}
/// getExact - Get the exact loop backedge taken count considering all loop
/// exits. If all exits are computable, this is the minimum computed count.
/// exits. A computable result can only be return for loops with a single exit.
/// Returning the minimum taken count among all exits is incorrect because one
/// of the loop's exit limit's may have been skipped. HowFarToZero assumes that
/// the limit of each loop test is never skipped. This is a valid assumption as
/// long as the loop exits via that test. For precise results, it is the
/// caller's responsibility to specify the relevant loop exit using
/// getExact(ExitingBlock, SE).
const SCEV *
ScalarEvolution::BackedgeTakenInfo::getExact(ScalarEvolution *SE) const {
// If any exits were not computable, the loop is not computable.
if (!ExitNotTaken.isCompleteList()) return SE->getCouldNotCompute();
// We need at least one computable exit.
// We need exactly one computable exit.
if (!ExitNotTaken.ExitingBlock) return SE->getCouldNotCompute();
assert(ExitNotTaken.ExactNotTaken && "uninitialized not-taken info");
@ -4171,8 +4177,8 @@ ScalarEvolution::BackedgeTakenInfo::getExact(ScalarEvolution *SE) const {
if (!BECount)
BECount = ENT->ExactNotTaken;
else
BECount = SE->getUMinFromMismatchedTypes(BECount, ENT->ExactNotTaken);
else if (BECount != ENT->ExactNotTaken)
return SE->getCouldNotCompute();
}
assert(BECount && "Invalid not taken count for loop exit");
return BECount;
@ -4253,8 +4259,15 @@ ScalarEvolution::ComputeBackedgeTakenCount(const Loop *L) {
if (MaxBECount == getCouldNotCompute())
MaxBECount = EL.Max;
else if (EL.Max != getCouldNotCompute())
MaxBECount = getUMinFromMismatchedTypes(MaxBECount, EL.Max);
else if (EL.Max != getCouldNotCompute()) {
// We cannot take the "min" MaxBECount, because non-unit stride loops may
// skip some loop tests. Taking the max over the exits is sufficiently
// conservative. TODO: We could do better taking into consideration
// that (1) the loop has unit stride (2) the last loop test is
// less-than/greater-than (3) any loop test is less-than/greater-than AND
// falls-through some constant times less then the other tests.
MaxBECount = getUMaxFromMismatchedTypes(MaxBECount, EL.Max);
}
}
return BackedgeTakenInfo(ExitCounts, CouldComputeBECount, MaxBECount);
@ -5507,10 +5520,10 @@ ScalarEvolution::HowFarToZero(const SCEV *V, const Loop *L) {
// behavior. Loops must exhibit defined behavior until a wrapped value is
// actually used. So the trip count computed by udiv could be smaller than the
// number of well-defined iterations.
if (AddRec->getNoWrapFlags(SCEV::FlagNW))
if (AddRec->getNoWrapFlags(SCEV::FlagNW)) {
// FIXME: We really want an "isexact" bit for udiv.
return getUDivExpr(Distance, CountDown ? getNegativeSCEV(Step) : Step);
}
// Then, try to solve the above equation provided that Start is constant.
if (const SCEVConstant *StartC = dyn_cast<SCEVConstant>(Start))
return SolveLinEquationWithOverflow(StepC->getValue()->getValue(),

40
test/Transforms/IndVarSimplify/2011-11-15-multiexit.ll Normal file
View File

@ -0,0 +1,40 @@
; RUN: opt < %s -indvars -S | FileCheck %s
;
; Prior to the fix for PR11375, indvars would replace %firstIV with a
; loop-invariant gep computed in the preheader. This was incorrect
; because it was based on the minimum "ExitNotTaken" count. If the
; final loop test is skipped (odd number of elements) then the early
; exit would be taken and the loop invariant value would be incorrect.
target datalayout = "e-p:64:64:64-i1:8:8-i8:8:8-i16:16:16-i32:32:32-i64:64:64-f32:32:32-f64:64:64-v64:64:64-v128:128:128-a0:0:64-s0:64:64-f80:128:128-n8:16:32:64-S128"
target triple = "x86_64-apple-darwin"
; CHECK: if.end:
; CHECK: phi i32* [ %first.lcssa, %early.exit ]
define i32 @test(i32* %first, i32* %last) uwtable ssp {
entry:
br i1 undef, label %if.end, label %if.then
if.then: ; preds = %entry
br i1 undef, label %if.end, label %do.body
do.body: ; preds = %if.else, %if.then
%firstIV = phi i32* [ %incdec.ptr2, %if.else ], [ %first, %if.then ]
%incdec.ptr1 = getelementptr inbounds i32* %firstIV, i64 1
%cmp1 = icmp eq i32* %incdec.ptr1, %last
br i1 %cmp1, label %early.exit, label %if.else
if.else: ; preds = %do.body
%incdec.ptr2 = getelementptr inbounds i32* %firstIV, i64 2
%cmp2 = icmp eq i32* %incdec.ptr2, %last
br i1 %cmp2, label %if.end, label %do.body
early.exit:
%first.lcssa = phi i32* [ %firstIV, %do.body ]
br label %if.end
if.end:
%tmp = phi i32* [ %first.lcssa, %early.exit ], [ %first, %if.then ], [ %first, %entry ], [ undef, %if.else ]
%val = load i32* %tmp
ret i32 %val
}

8
test/Transforms/IndVarSimplify/loop_evaluate10.ll
View File

@ -1,8 +1,14 @@
; RUN: opt < %s -indvars -S \
; RUN: | grep {%b.1 = phi i32 \\\[ 2, %bb \\\], \\\[ 1, %bb2 \\\]}
;
; This loop has multiple exits, and the value of %b1 depends on which
; exit is taken. Indvars should correctly compute the exit values.
;
; XFAIL: *
; Indvars does not currently replace loop invariant values unless all
; loop exits have the same exit value. We could handle some cases,
; such as this, by making getSCEVAtScope() sensitive to a particular
; loop exit. See PR11388.
target datalayout = "e-p:64:64:64-i1:8:8-i8:8:8-i16:16:16-i32:32:32-i64:64:64-f32:32:32-f64:64:64-v64:64:64-v128:128:128-a0:0:64-s0:64:64-f80:128:128"
target triple = "x86_64-pc-linux-gnu"

7
test/Transforms/IndVarSimplify/loop_evaluate9.ll
View File

@ -2,8 +2,13 @@
; RUN: grep {\[%\]tmp7 = icmp eq i8 -28, -28} %t
; RUN: grep {\[%\]tmp8 = icmp eq i8 63, 63} %t
; PR4477
; Indvars should compute the exit values in loop.
;
; XFAIL: *
; Indvars does not currently replace loop invariant values unless all
; loop exits have the same exit value. We could handle some cases,
; such as this, by making getSCEVAtScope() sensitive to a particular
; loop exit. See PR11388.
target datalayout = "e-p:32:32:32-i1:8:8-i8:8:8-i16:16:16-i32:32:32-i64:32:64-f32:32:32-f64:32:64-v64:64:64-v128:128:128-a0:0:64-f80:32:32"
target triple = "i386-pc-linux-gnu"