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[LAA] LLE 2/6: Fix a NoDep case that should be a Forward dependence

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Summary:
When the dependence distance in zero then we have a loop-independent
dependence from the earlier to the later access.

No current client of LAA uses forward dependences so other than
potentially hitting the MaxDependences threshold earlier, this change
shouldn't affect anything right now.

This and the previous patch were tested together for compile-time
regression.  None found in LNT/SPEC.

Reviewers: hfinkel

Subscribers: rengolin, llvm-commits

Differential Revision: http://reviews.llvm.org/D13255

llvm-svn: 251973
This commit is contained in:
Adam Nemet 2015-11-03 20:13:43 +00:00
parent c168f14a53
commit b9c59b29d9
3 changed files with 73 additions and 1 deletions
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8
include/llvm/Analysis/LoopAccessAnalysis.h
View File

@ -136,6 +136,14 @@ public:
// We couldn't determine the direction or the distance.
Unknown,
// Lexically forward.
//
// FIXME: If we only have loop-independent forward dependences (e.g. a
// read and write of A[i]), LAA will locally deem the dependence "safe"
// without querying the MemoryDepChecker. Therefore we can miss
// enumerating loop-independent forward dependences in
// getInterestingDependences. Note that as soon as there are different
// indices used to access the same array, the MemoryDepChecker *is*
// queried and the dependence list is complete.
Forward,
// Forward, but if vectorized, is likely to prevent store-to-load
// forwarding.

2
lib/Analysis/LoopAccessAnalysis.cpp
View File

@ -1090,7 +1090,7 @@ MemoryDepChecker::isDependent(const MemAccessInfo &A, unsigned AIdx,
// Could be improved to assert type sizes are the same (i32 == float, etc).
if (Val == 0) {
if (ATy == BTy)
return Dependence::NoDep;
return Dependence::Forward;
DEBUG(dbgs() << "LAA: Zero dependence difference but different types\n");
return Dependence::Unknown;
}

64
test/Analysis/LoopAccessAnalysis/forward-loop-independent.ll Normal file
View File

@ -0,0 +1,64 @@
; RUN: opt -loop-accesses -analyze < %s | FileCheck %s
; Check that loop-indepedent forward dependences are discovered properly.
;
; FIXME: This does not actually always work which is pretty confusing. Right
; now there is hack in LAA that tries to figure out loop-indepedent forward
; dependeces *outside* of the MemoryDepChecker logic (i.e. proper dependence
; analysis).
;
; Therefore if there is only loop-independent dependences for an array
; (i.e. the same index is used), we don't discover the forward dependence.
; So, at ***, we add another non-I-based access of A to trigger
; MemoryDepChecker analysis for accesses of A.
;
; for (unsigned i = 0; i < 100; i++) {
; A[i + 1] = B[i] + 1; // ***
; A[i] = B[i] + 2;
; C[i] = A[i] * 2;
; }
target datalayout = "e-m:o-i64:64-f80:128-n8:16:32:64-S128"
define void @f(i32* noalias %A, i32* noalias %B, i32* noalias %C, i64 %N) {
; CHECK: Interesting Dependences:
; CHECK-NEXT: Forward:
; CHECK-NEXT: store i32 %b_p1, i32* %Aidx, align 4 ->
; CHECK-NEXT: %a = load i32, i32* %Aidx, align 4
; CHECK: ForwardButPreventsForwarding:
; CHECK-NEXT: store i32 %b_p2, i32* %Aidx_next, align 4 ->
; CHECK-NEXT: %a = load i32, i32* %Aidx, align 4
; CHECK: Forward:
; CHECK-NEXT: store i32 %b_p2, i32* %Aidx_next, align 4 ->
; CHECK-NEXT: store i32 %b_p1, i32* %Aidx, align 4
entry:
br label %for.body
for.body: ; preds = %for.body, %entry
%indvars.iv = phi i64 [ 0, %entry ], [ %indvars.iv.next, %for.body ]
%indvars.iv.next = add nuw nsw i64 %indvars.iv, 1
%Bidx = getelementptr inbounds i32, i32* %B, i64 %indvars.iv
%Cidx = getelementptr inbounds i32, i32* %C, i64 %indvars.iv
%Aidx_next = getelementptr inbounds i32, i32* %A, i64 %indvars.iv.next
%Aidx = getelementptr inbounds i32, i32* %A, i64 %indvars.iv
%b = load i32, i32* %Bidx, align 4
%b_p2 = add i32 %b, 1
store i32 %b_p2, i32* %Aidx_next, align 4
%b_p1 = add i32 %b, 2
store i32 %b_p1, i32* %Aidx, align 4
%a = load i32, i32* %Aidx, align 4
%c = mul i32 %a, 2
store i32 %c, i32* %Cidx, align 4
%exitcond = icmp eq i64 %indvars.iv.next, %N
br i1 %exitcond, label %for.end, label %for.body
for.end: ; preds = %for.body
ret void
}