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BasicAA: Recognize cyclic NoAlias phis

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Enhances basic alias analysis to recognize phis whose first incoming values are
NoAlias and whose other incoming values are just the phi node itself through
some amount of recursion.

Example: With this change basicaa reports that ptr_phi and ptr_phi2 do not alias
each other.

bb:
 ptr = ptr2 + 1

loop:
  ptr_phi = phi [bb, ptr], [loop, ptr_plus_one]
  ptr2_phi = phi [bb, ptr2], [loop, ptr2_plus_one]
  ...
  ptr_plus_one = gep ptr_phi, 1
  ptr2_plus_one = gep ptr2_phi, 1

This enables the elimination of one load in code like the following:

extern int foo;

int test_noalias(int *ptr, int num, int* coeff) {
  int *ptr2 = ptr;
  int result = (*ptr++) * (*coeff--);
  while (num--) {
    *ptr2++ = *ptr;
    result +=  (*coeff--) * (*ptr++);
  }
  *ptr = foo;
  return result;
}

Part 2/2 of fix for PR13564.


git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@163319 91177308-0d34-0410-b5e6-96231b3b80d8
This commit is contained in:
Arnold Schwaighofer 2012-09-06 14:41:53 +00:00
parent 64eacd9136
commit 3d5f96ee1b
2 changed files with 68 additions and 0 deletions
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35
lib/Analysis/BasicAliasAnalysis.cpp
View File

@ -1060,12 +1060,42 @@ BasicAliasAnalysis::aliasPHI(const PHINode *PN, uint64_t PNSize,
// on corresponding edges.
if (const PHINode *PN2 = dyn_cast<PHINode>(V2))
if (PN2->getParent() == PN->getParent()) {
LocPair Locs(Location(PN, PNSize, PNTBAAInfo),
Location(V2, V2Size, V2TBAAInfo));
if (PN > V2)
std::swap(Locs.first, Locs.second);
AliasResult Alias =
aliasCheck(PN->getIncomingValue(0), PNSize, PNTBAAInfo,
PN2->getIncomingValueForBlock(PN->getIncomingBlock(0)),
V2Size, V2TBAAInfo);
if (Alias == MayAlias)
return MayAlias;
// If the first source of the PHI nodes NoAlias and the other inputs are
// the PHI node itself through some amount of recursion this does not add
// any new information so just return NoAlias.
// bb:
// ptr = ptr2 + 1
// loop:
// ptr_phi = phi [bb, ptr], [loop, ptr_plus_one]
// ptr2_phi = phi [bb, ptr2], [loop, ptr2_plus_one]
// ...
// ptr_plus_one = gep ptr_phi, 1
// ptr2_plus_one = gep ptr2_phi, 1
// We assume for the recursion that the the phis (ptr_phi, ptr2_phi) do
// not alias each other.
bool ArePhisAssumedNoAlias = false;
AliasResult OrigAliasResult;
if (Alias == NoAlias) {
// Pretend the phis do not alias.
assert(AliasCache.count(Locs) &&
"There must exist an entry for the phi node");
OrigAliasResult = AliasCache[Locs];
AliasCache[Locs] = NoAlias;
ArePhisAssumedNoAlias = true;
}
for (unsigned i = 1, e = PN->getNumIncomingValues(); i != e; ++i) {
AliasResult ThisAlias =
aliasCheck(PN->getIncomingValue(i), PNSize, PNTBAAInfo,
@ -1075,6 +1105,11 @@ BasicAliasAnalysis::aliasPHI(const PHINode *PN, uint64_t PNSize,
if (Alias == MayAlias)
break;
}
// Reset if speculation failed.
if (ArePhisAssumedNoAlias && Alias != NoAlias)
AliasCache[Locs] = OrigAliasResult;
return Alias;
}

33
test/Analysis/BasicAA/phi-speculation.ll Normal file
View File

@ -0,0 +1,33 @@
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"
; RUN: opt < %s -basicaa -aa-eval -print-all-alias-modref-info -disable-output 2>&1 | FileCheck %s
; ptr_phi and ptr2_phi do not alias.
; CHECK: NoAlias: i32* %ptr2_phi, i32* %ptr_phi
define i32 @test_noalias(i32* %ptr2, i32 %count, i32* %coeff) {
entry:
%ptr = getelementptr inbounds i32* %ptr2, i64 1
br label %while.body
while.body:
%num = phi i32 [ %count, %entry ], [ %dec, %while.body ]
%ptr_phi = phi i32* [ %ptr, %entry ], [ %ptr_inc, %while.body ]
%ptr2_phi = phi i32* [ %ptr2, %entry ], [ %ptr2_inc, %while.body ]
%result.09 = phi i32 [ 0 , %entry ], [ %add, %while.body ]
%dec = add nsw i32 %num, -1
%0 = load i32* %ptr_phi, align 4
store i32 %0, i32* %ptr2_phi, align 4
%1 = load i32* %coeff, align 4
%2 = load i32* %ptr_phi, align 4
%mul = mul nsw i32 %1, %2
%add = add nsw i32 %mul, %result.09
%tobool = icmp eq i32 %dec, 0
%ptr_inc = getelementptr inbounds i32* %ptr_phi, i64 1
%ptr2_inc = getelementptr inbounds i32* %ptr2_phi, i64 1
br i1 %tobool, label %the_exit, label %while.body
the_exit:
ret i32 %add
}