maxjhandsome
/
llvm-project
forked from OSchip/llvm-project
1
0
Fork 0
Code Issues Pull Requests Packages Releases Wiki Activity

Assume GetElementPtr offsets to be inbounds 

In case a GEP instruction references into a fixed size array e.g., an access
A[i][j] into an array A[100x100], LLVM-IR does not guarantee that the subscripts
always compute values that are within array bounds. We now derive the set of
parameter values for which all accesses are within bounds and add the assumption
that the scop is only every executed with this set of parameter values.

Example:

void foo(float A[][20], long n, long m {
    for (long i = 0; i < n; i++)
      for (long j = 0; j < m; j++)
        A[i][j] = ...

This loop yields out-of-bound accesses if m is at least 20 and at the same time
at least one iteration of the outer loop is executed. Hence, we assume:

  n <= 0 or m <= 20.

Doing so simplifies the dependence analysis problem, allows us to perform
more optimizations and generate better code.

TODO: The location where the GEP instruction is executed is not necessarily the
location where the memory is actually accessed. As a result scanning for GEP[s]
is imprecise. Even though this is not a correctness problem, this imprecision
may result in missed optimizations or non-optimal run-time checks.

In polybench where this mismatch between parametric loop bounds and fixed size
arrays is common, we see with this patch significant reductions in compile time
(up to 50%) and execution time (up to 70%). We see two significant compile time
regressions (fdtd-2d, jacobi-2d-imper), and one execution time regression
(trmm).  Both regressions arise due to additional optimizations that have been
enabled by this patch. They can be addressed in subsequent commits.

http://reviews.llvm.org/D6369

llvm-svn: 222754
This commit is contained in:
Tobias Grosser 2014-11-25 10:51:12 +00:00
parent 4c8cf4f7bc
commit 7b50beebe4
10 changed files with 283 additions and 32 deletions
Show Stats Download Patch File Download Diff File Expand all files Collapse all files

32
polly/include/polly/ScopInfo.h
View File

@ -455,6 +455,38 @@ class ScopStmt {
llvm::SmallVectorImpl<MemoryAccess *> &Loads);
//@}
/// @brief Derive assumptions about parameter values from GetElementPtrInst
///
/// In case a GEP instruction references into a fixed size array e.g., an
/// access A[i][j] into an array A[100x100], LLVM-IR does not guarantee that
/// the subscripts always compute values that are within array bounds. In this
/// function we derive the set of parameter values for which all accesses are
/// within bounds and add the assumption that the scop is only every executed
/// with this set of parameter values.
///
/// Example:
///
/// void foo(float A[][20], long n, long m {
/// for (long i = 0; i < n; i++)
/// for (long j = 0; j < m; j++)
/// A[i][j] = ...
///
/// This loop yields out-of-bound accesses if m is at least 20 and at the same
/// time at least one iteration of the outer loop is executed. Hence, we
/// assume:
///
/// n <= 0 or m <= 20.
///
/// TODO: The location where the GEP instruction is executed is not
/// necessarily the location where the memory is actually accessed. As a
/// result scanning for GEP[s] is imprecise. Even though this is not a
/// correctness problem, this imprecision may result in missed optimizations
/// or non-optimal run-time checks.
void deriveAssumptionsFromGEP(GetElementPtrInst *Inst);
/// @brief Scan the scop and derive assumptions about parameter values.
void deriveAssumptions();
/// Create the ScopStmt from a BasicBlock.
ScopStmt(Scop &parent, TempScop &tempScop, const Region &CurRegion,
BasicBlock &bb, SmallVectorImpl<Loop *> &NestLoops,

3
polly/lib/Analysis/Dependences.cpp
View File

@ -122,6 +122,9 @@ void Dependences::collectInfo(Scop &S, isl_union_map **Read,
}
*StmtSchedule = isl_union_map_add_map(*StmtSchedule, Stmt->getScattering());
}
*StmtSchedule =
isl_union_map_intersect_params(*StmtSchedule, S.getAssumedContext());
}
/// @brief Fix all dimension of @p Zero to 0 and add it to @p user

57
polly/lib/Analysis/ScopInfo.cpp
View File

@ -847,6 +847,61 @@ __isl_give isl_set *ScopStmt::buildDomain(TempScop &tempScop,
return Domain;
}
void ScopStmt::deriveAssumptionsFromGEP(GetElementPtrInst *GEP) {
int Dimension = 0;
isl_ctx *Ctx = Parent.getIslCtx();
isl_local_space *LSpace = isl_local_space_from_space(getDomainSpace());
Type *Ty = GEP->getPointerOperandType();
ScalarEvolution &SE = *Parent.getSE();
if (auto *PtrTy = dyn_cast<PointerType>(Ty)) {
Dimension = 1;
Ty = PtrTy->getElementType();
}
while (auto ArrayTy = dyn_cast<ArrayType>(Ty)) {
unsigned int Operand = 1 + Dimension;
if (GEP->getNumOperands() <= Operand)
break;
const SCEV *Expr = SE.getSCEV(GEP->getOperand(Operand));
if (isAffineExpr(&Parent.getRegion(), Expr, SE)) {
isl_pw_aff *AccessOffset = SCEVAffinator::getPwAff(this, Expr);
AccessOffset =
isl_pw_aff_set_tuple_id(AccessOffset, isl_dim_in, getDomainId());
isl_pw_aff *DimSize = isl_pw_aff_from_aff(isl_aff_val_on_domain(
isl_local_space_copy(LSpace),
isl_val_int_from_si(Ctx, ArrayTy->getNumElements())));
isl_set *OutOfBound = isl_pw_aff_ge_set(AccessOffset, DimSize);
OutOfBound = isl_set_intersect(getDomain(), OutOfBound);
OutOfBound = isl_set_params(OutOfBound);
isl_set *InBound = isl_set_complement(OutOfBound);
isl_set *Executed = isl_set_params(getDomain());
// A => B == !A or B
isl_set *InBoundIfExecuted =
isl_set_union(isl_set_complement(Executed), InBound);
Parent.addAssumption(InBoundIfExecuted);
}
Dimension += 1;
Ty = ArrayTy->getElementType();
}
isl_local_space_free(LSpace);
}
void ScopStmt::deriveAssumptions() {
for (Instruction &Inst : *BB)
if (auto *GEP = dyn_cast<GetElementPtrInst>(&Inst))
deriveAssumptionsFromGEP(GEP);
}
ScopStmt::ScopStmt(Scop &parent, TempScop &tempScop, const Region &CurRegion,
BasicBlock &bb, SmallVectorImpl<Loop *> &Nest,
SmallVectorImpl<unsigned> &Scatter)
@ -867,6 +922,7 @@ ScopStmt::ScopStmt(Scop &parent, TempScop &tempScop, const Region &CurRegion,
buildScattering(Scatter);
buildAccesses(tempScop);
checkForReductions();
deriveAssumptions();
}
/// @brief Collect loads which might form a reduction chain with @p StoreMA
@ -1530,6 +1586,7 @@ __isl_give isl_set *Scop::getAssumedContext() const {
void Scop::addAssumption(__isl_take isl_set *Set) {
AssumedContext = isl_set_intersect(AssumedContext, Set);
AssumedContext = isl_set_coalesce(AssumedContext);
}
void Scop::printContext(raw_ostream &OS) const {

2
polly/test/Dependences/sequential_loops.ll
View File

@ -273,7 +273,7 @@ exit.2:
; VALUE: RAW dependences:
; VALUE: [p] -> {
; VALUE: Stmt_S1[i0] -> Stmt_S2[-p + i0] :
; VALUE: i0 >= p and i0 <= 9 + p and i0 >= 0 and i0 <= 99
; VALUE: i0 >= p and i0 <= 9 + p and p <= 190 and i0 <= 99 and i0 >= 0
; VALUE: }
; VALUE: WAR dependences:
; VALUE: [p] -> {

21
polly/test/Isl/Ast/OpenMP/nested_loop_both_parallel_parametric.ll
View File

@ -41,20 +41,9 @@ ret:
ret void
}
; At the first look both loops seem parallel, however due to the linearization
; of memory access functions, we get the following dependences:
; [n] -> { loop_body[i0, i1] -> loop_body[1024 + i0, -1 + i1]:
; 0 <= i0 < n - 1024 and 1 <= i1 < n}
; They cause the outer loop to be non-parallel. We can only prove their
; absence, if we know that n < 1024. This information is currently not available
; to polly. However, we should be able to obtain it due to the out of bounds
; memory accesses, that would happen if n >= 1024.
; Note that we do not delinearize this access function because it is considered
; to already be affine: {{0,+,4}<%loop.i>,+,4096}<%loop.j>.
; CHECK: for (int c1 = 0; c1 < n; c1 += 1)
; CHECK: #pragma simd
; CHECK: if (n <= 1024 ? 1 : 0)
; CHECK: #pragma omp parallel for
; CHECK: for (int c3 = 0; c3 < n; c3 += 1)
; CHECK: Stmt_loop_body(c1, c3);
; CHECK: for (int c1 = 0; c1 < n; c1 += 1)
; CHECK: #pragma simd
; CHECK: for (int c3 = 0; c3 < n; c3 += 1)
; CHECK: Stmt_loop_body(c1, c3);

10
polly/test/Isl/Ast/alias_simple_1.ll
View File

@ -12,11 +12,11 @@
; A[i] = B[i];
; }
;
; NOAA: if (1 && (&MemRef_A[N] <= &MemRef_B[0] || &MemRef_B[N] <= &MemRef_A[0]))
; BASI: if (1 && (&MemRef_A[N] <= &MemRef_B[0] || &MemRef_B[N] <= &MemRef_A[0]))
; TBAA: if (1)
; SCEV: if (1 && (&MemRef_A[N] <= &MemRef_B[0] || &MemRef_B[N] <= &MemRef_A[0]))
; GLOB: if (1 && (&MemRef_A[N] <= &MemRef_B[0] || &MemRef_B[N] <= &MemRef_A[0]))
; NOAA: if ((N <= 1024 ? 1 : 0) && (&MemRef_A[N] <= &MemRef_B[0] || &MemRef_B[N] <= &MemRef_A[0]))
; BASI: if ((N <= 1024 ? 1 : 0) && (&MemRef_A[N] <= &MemRef_B[0] || &MemRef_B[N] <= &MemRef_A[0]))
; TBAA: if (N <= 1024 ? 1 : 0)
; SCEV: if ((N <= 1024 ? 1 : 0) && (&MemRef_A[N] <= &MemRef_B[0] || &MemRef_B[N] <= &MemRef_A[0]))
; GLOB: if ((N <= 1024 ? 1 : 0) && (&MemRef_A[N] <= &MemRef_B[0] || &MemRef_B[N] <= &MemRef_A[0]))
;
target datalayout = "e-m:e-i64:64-f80:128-n8:16:32:64-S128"

10
polly/test/Isl/Ast/alias_simple_2.ll
View File

@ -12,11 +12,11 @@
; A[i] = B[i];
; }
;
; NOAA: if (1 && (&MemRef_A[N] <= &MemRef_B[0] || &MemRef_B[N] <= &MemRef_A[0]))
; BASI: if (1)
; TBAA: if (1 && (&MemRef_A[N] <= &MemRef_B[0] || &MemRef_B[N] <= &MemRef_A[0]))
; SCEV: if (1 && (&MemRef_A[N] <= &MemRef_B[0] || &MemRef_B[N] <= &MemRef_A[0]))
; GLOB: if (1 && (&MemRef_A[N] <= &MemRef_B[0] || &MemRef_B[N] <= &MemRef_A[0]))
; NOAA: if ((N <= 1024 ? 1 : 0) && (&MemRef_A[N] <= &MemRef_B[0] || &MemRef_B[N] <= &MemRef_A[0]))
; BASI: if (N <= 1024 ? 1 : 0)
; TBAA: if ((N <= 1024 ? 1 : 0) && (&MemRef_A[N] <= &MemRef_B[0] || &MemRef_B[N] <= &MemRef_A[0]))
; SCEV: if ((N <= 1024 ? 1 : 0) && (&MemRef_A[N] <= &MemRef_B[0] || &MemRef_B[N] <= &MemRef_A[0]))
; GLOB: if ((N <= 1024 ? 1 : 0) && (&MemRef_A[N] <= &MemRef_B[0] || &MemRef_B[N] <= &MemRef_A[0]))
;
target datalayout = "e-m:e-i64:64-f80:128-n8:16:32:64-S128"

10
polly/test/Isl/Ast/alias_simple_3.ll
View File

@ -12,11 +12,11 @@
; A[i] = B[i];
; }
;
; NOAA: if (1 && (&MemRef_A[N] <= &MemRef_B[0] || &MemRef_B[N] <= &MemRef_A[0]))
; BASI: if (1)
; TBAA: if (1)
; SCEV: if (1 && (&MemRef_A[N] <= &MemRef_B[0] || &MemRef_B[N] <= &MemRef_A[0]))
; GLOB: if (1 && (&MemRef_A[N] <= &MemRef_B[0] || &MemRef_B[N] <= &MemRef_A[0]))
; NOAA: if ((N <= 1024 ? 1 : 0) && (&MemRef_A[N] <= &MemRef_B[0] || &MemRef_B[N] <= &MemRef_A[0]))
; BASI: if (N <= 1024 ? 1 : 0)
; TBAA: if (N <= 1024 ? 1 : 0)
; SCEV: if ((N <= 1024 ? 1 : 0) && (&MemRef_A[N] <= &MemRef_B[0] || &MemRef_B[N] <= &MemRef_A[0]))
; GLOB: if ((N <= 1024 ? 1 : 0) && (&MemRef_A[N] <= &MemRef_B[0] || &MemRef_B[N] <= &MemRef_A[0]))
;
target datalayout = "e-m:e-i64:64-f80:128-n8:16:32:64-S128"

76
polly/test/ScopInfo/assume_gep_bounds.ll Normal file
View File

@ -0,0 +1,76 @@
; RUN: opt %loadPolly -polly-scops -analyze < %s | FileCheck %s
; void foo(float A[][20][30], long n, long m, long p) {
; for (long i = 0; i < n; i++)
; for (long j = 0; j < m; j++)
; for (long k = 0; k < p; k++)
; A[i][j][k] = i + j + k;
; }
; For the above code we want to assume that all memory accesses are within the
; bounds of the array A. In C (and LLVM-IR) this is not required, such that out
; of bounds accesses are valid. However, as such accesses are uncommon, cause
; complicated dependence pattern and as a result make dependence analysis more
; costly and may prevent or hinder useful program transformations, we assume
; absence of out-of-bound accesses. To do so we derive the set of parameter
; values for which our assumption holds.
; CHECK: Assumed Context
; CHECK-NEXT: [n, m, p] -> { : p <= 30 and m <= 20 }
target datalayout = "e-m:e-i64:64-f80:128-n8:16:32:64-S128"
define void @foo([20 x [30 x float]]* %A, i64 %n, i64 %m, i64 %p) {
entry:
br label %for.cond
for.cond: ; preds = %for.inc13, %entry
%i.0 = phi i64 [ 0, %entry ], [ %inc14, %for.inc13 ]
%cmp = icmp slt i64 %i.0, %n
br i1 %cmp, label %for.body, label %for.end15
for.body: ; preds = %for.cond
br label %for.cond1
for.cond1: ; preds = %for.inc10, %for.body
%j.0 = phi i64 [ 0, %for.body ], [ %inc11, %for.inc10 ]
%cmp2 = icmp slt i64 %j.0, %m
br i1 %cmp2, label %for.body3, label %for.end12
for.body3: ; preds = %for.cond1
br label %for.cond4
for.cond4: ; preds = %for.inc, %for.body3
%k.0 = phi i64 [ 0, %for.body3 ], [ %inc, %for.inc ]
%cmp5 = icmp slt i64 %k.0, %p
br i1 %cmp5, label %for.body6, label %for.end
for.body6: ; preds = %for.cond4
%add = add nsw i64 %i.0, %j.0
%add7 = add nsw i64 %add, %k.0
%conv = sitofp i64 %add7 to float
%arrayidx9 = getelementptr inbounds [20 x [30 x float]]* %A, i64 %i.0, i64 %j.0, i64 %k.0
store float %conv, float* %arrayidx9, align 4
br label %for.inc
for.inc: ; preds = %for.body6
%inc = add nsw i64 %k.0, 1
br label %for.cond4
for.end: ; preds = %for.cond4
br label %for.inc10
for.inc10: ; preds = %for.end
%inc11 = add nsw i64 %j.0, 1
br label %for.cond1
for.end12: ; preds = %for.cond1
br label %for.inc13
for.inc13: ; preds = %for.end12
%inc14 = add nsw i64 %i.0, 1
br label %for.cond
for.end15: ; preds = %for.cond
ret void
}

94
polly/test/ScopInfo/assume_gep_bounds_2.ll Normal file
View File

@ -0,0 +1,94 @@
; RUN: opt %loadPolly -basicaa -polly-scops -analyze < %s | FileCheck %s
;
; void foo(float A[restrict][20], float B[restrict][20], long n, long m,
; long p) {
; for (long i = 0; i < n; i++)
; for (long j = 0; j < m; j++)
; A[i][j] = i + j;
; for (long i = 0; i < m; i++)
; for (long j = 0; j < p; j++)
; B[i][j] = i + j;
; }
; This code is within bounds either if m and p are smaller than the array sizes,
; but also if only p is smaller than the size of the second B dimension and n
; is such that the first loop is never executed and consequently A is never
; accessed. In this case the value of m does not matter.
; CHECK: Assumed Context:
; CHECK-NEXT: [n, m, p] -> { : (n <= 0 and p <= 20) or (m <= 20 and p <= 20) }
target datalayout = "e-m:e-i64:64-f80:128-n8:16:32:64-S128"
define void @foo([20 x float]* noalias %A, [20 x float]* noalias %B, i64 %n, i64 %m, i64 %p) {
entry:
br label %for.cond
for.cond: ; preds = %for.inc5, %entry
%i.0 = phi i64 [ 0, %entry ], [ %inc6, %for.inc5 ]
%cmp = icmp slt i64 %i.0, %n
br i1 %cmp, label %for.body, label %for.end7
for.body: ; preds = %for.cond
br label %for.cond1
for.cond1: ; preds = %for.inc, %for.body
%j.0 = phi i64 [ 0, %for.body ], [ %inc, %for.inc ]
%cmp2 = icmp slt i64 %j.0, %m
br i1 %cmp2, label %for.body3, label %for.end
for.body3: ; preds = %for.cond1
%add = add nsw i64 %i.0, %j.0
%conv = sitofp i64 %add to float
%arrayidx4 = getelementptr inbounds [20 x float]* %A, i64 %i.0, i64 %j.0
store float %conv, float* %arrayidx4, align 4
br label %for.inc
for.inc: ; preds = %for.body3
%inc = add nsw i64 %j.0, 1
br label %for.cond1
for.end: ; preds = %for.cond1
br label %for.inc5
for.inc5: ; preds = %for.end
%inc6 = add nsw i64 %i.0, 1
br label %for.cond
for.end7: ; preds = %for.cond
br label %for.cond9
for.cond9: ; preds = %for.inc25, %for.end7
%i8.0 = phi i64 [ 0, %for.end7 ], [ %inc26, %for.inc25 ]
%cmp10 = icmp slt i64 %i8.0, %m
br i1 %cmp10, label %for.body12, label %for.end27
for.body12: ; preds = %for.cond9
br label %for.cond14
for.cond14: ; preds = %for.inc22, %for.body12
%j13.0 = phi i64 [ 0, %for.body12 ], [ %inc23, %for.inc22 ]
%cmp15 = icmp slt i64 %j13.0, %p
br i1 %cmp15, label %for.body17, label %for.end24
for.body17: ; preds = %for.cond14
%add18 = add nsw i64 %i8.0, %j13.0
%conv19 = sitofp i64 %add18 to float
%arrayidx21 = getelementptr inbounds [20 x float]* %B, i64 %i8.0, i64 %j13.0
store float %conv19, float* %arrayidx21, align 4
br label %for.inc22
for.inc22: ; preds = %for.body17
%inc23 = add nsw i64 %j13.0, 1
br label %for.cond14
for.end24: ; preds = %for.cond14
br label %for.inc25
for.inc25: ; preds = %for.end24
%inc26 = add nsw i64 %i8.0, 1
br label %for.cond9
for.end27: ; preds = %for.cond9
ret void
}