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Stabilize alloca slices sort in SROA

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Slice::operator<() has a non-deterministic behavior. If we have
identical slices comparison will depend on the order or operands.
Normally that does not result in unstable compilation results
because the order in which slices are inserted into the vector
is deterministic and llvm::sort() normally behaves as a stable
sort, although that is not guaranteed.

However, there is test option -sroa-random-shuffle-slices which
is used to check exactly this aspect. The vector is first randomly
shuffled and then sorted. The same shuffling happens without this
option under expensive llvm checks.

I have managed to write a test which has hit this problem.

There are no fields in the Slice class to resolve the instability.
We only have offsets, IsSplittable and Use, but neither Use nor
User have anything suitable for predictable comparison.

I have switched to stable_sort which has to be sufficient and
removed that randon shuffle option.

Differential Revision: https://reviews.llvm.org/D81310
This commit is contained in:
Stanislav Mekhanoshin 2020-06-08 08:27:07 -07:00
parent 89848a578b
commit 446c39a70c
2 changed files with 50 additions and 19 deletions
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20
lib/Transforms/Scalar/SROA.cpp
View File

@ -94,11 +94,6 @@
#include <utility>
#include <vector>
#ifndef NDEBUG
// We only use this for a debug check.
#include <random>
#endif
using namespace llvm;
using namespace llvm::sroa;
@ -115,11 +110,6 @@ STATISTIC(NumLoadsSpeculated, "Number of loads speculated to allow promotion");
STATISTIC(NumDeleted, "Number of instructions deleted");
STATISTIC(NumVectorized, "Number of vectorized aggregates");
/// Hidden option to enable randomly shuffling the slices to help uncover
/// instability in their order.
static cl::opt<bool> SROARandomShuffleSlices("sroa-random-shuffle-slices",
cl::init(false), cl::Hidden);
/// Hidden option to experiment with completely strict handling of inbounds
/// GEPs.
static cl::opt<bool> SROAStrictInbounds("sroa-strict-inbounds", cl::init(false),
@ -1071,17 +1061,9 @@ AllocaSlices::AllocaSlices(const DataLayout &DL, AllocaInst &AI)
llvm::remove_if(Slices, [](const Slice &S) { return S.isDead(); }),
Slices.end());
#ifndef NDEBUG
if (SROARandomShuffleSlices) {
std::mt19937 MT(static_cast<unsigned>(
std::chrono::system_clock::now().time_since_epoch().count()));
std::shuffle(Slices.begin(), Slices.end(), MT);
}
#endif
// Sort the uses. This arranges for the offsets to be in ascending order,
// and the sizes to be in descending order.
llvm::sort(Slices);
std::stable_sort(Slices.begin(), Slices.end());
}
#if !defined(NDEBUG) || defined(LLVM_ENABLE_DUMP)

49
test/Transforms/SROA/phi-gep.ll
View File

@ -367,6 +367,55 @@ exit:
unreachable
}
define i32 @test_sroa_gep_cast_phi_gep(i1 %cond) {
; CHECK-LABEL: @test_sroa_gep_cast_phi_gep(
; CHECK-NEXT: entry:
; CHECK-NEXT: [[A_SROA_0:%.*]] = alloca i32, align 4
; CHECK-NEXT: [[A_SROA_0_0_GEP_A_CAST_TO_I32_SROA_CAST:%.*]] = bitcast i32* [[A_SROA_0]] to float*
; CHECK-NEXT: [[A_SROA_0_0_GEP_A_CAST_TO_I32_SROA_CAST2:%.*]] = bitcast i32* [[A_SROA_0]] to float*
; CHECK-NEXT: [[A_SROA_0_0_GEP_SROA_CAST:%.*]] = bitcast i32* [[A_SROA_0]] to float*
; CHECK-NEXT: store i32 1065353216, i32* [[A_SROA_0]], align 4
; CHECK-NEXT: br i1 [[COND:%.*]], label [[FOR:%.*]], label [[END:%.*]]
; CHECK: for:
; CHECK-NEXT: [[PHI_I:%.*]] = phi i32 [ 0, [[ENTRY:%.*]] ], [ [[I:%.*]], [[FOR]] ]
; CHECK-NEXT: [[PHI:%.*]] = phi float* [ [[A_SROA_0_0_GEP_A_CAST_TO_I32_SROA_CAST]], [[ENTRY]] ], [ [[GEP_FOR_CAST_TO_I32:%.*]], [[FOR]] ]
; CHECK-NEXT: [[PHI_SROA_PHI:%.*]] = phi float* [ [[A_SROA_0_0_GEP_SROA_CAST]], [[ENTRY]] ], [ [[GEP_FOR_CAST_TO_I32_SROA_GEP:%.*]], [[FOR]] ]
; CHECK-NEXT: [[I]] = add i32 [[PHI_I]], 1
; CHECK-NEXT: [[GEP_FOR_CAST:%.*]] = bitcast float* [[PHI_SROA_PHI]] to i32*
; CHECK-NEXT: [[GEP_FOR_CAST_TO_I32]] = bitcast i32* [[GEP_FOR_CAST]] to float*
; CHECK-NEXT: [[GEP_FOR_CAST_TO_I32_SROA_GEP]] = getelementptr inbounds float, float* [[GEP_FOR_CAST_TO_I32]], i32 0
; CHECK-NEXT: [[LOOP_COND:%.*]] = icmp ult i32 [[I]], 10
; CHECK-NEXT: br i1 [[LOOP_COND]], label [[FOR]], label [[END]]
; CHECK: end:
; CHECK-NEXT: [[PHI_END:%.*]] = phi float* [ [[A_SROA_0_0_GEP_A_CAST_TO_I32_SROA_CAST2]], [[ENTRY]] ], [ [[PHI]], [[FOR]] ]
; CHECK-NEXT: [[PHI_END_1:%.*]] = bitcast float* [[PHI_END]] to i32*
; CHECK-NEXT: [[LOAD:%.*]] = load i32, i32* [[PHI_END_1]], align 4
; CHECK-NEXT: ret i32 [[LOAD]]
;
entry:
%a = alloca %pair, align 4
%gep_a = getelementptr inbounds %pair, %pair* %a, i32 0, i32 1
%gep_a_cast_to_float = bitcast i32* %gep_a to float*
store float 1.0, float* %gep_a_cast_to_float, align 4
br i1 %cond, label %for, label %end
for:
%phi_i = phi i32 [ 0, %entry ], [ %i, %for ]
%phi = phi float* [ %gep_a_cast_to_float, %entry], [ %gep_for_cast_to_float, %for ]
%i = add i32 %phi_i, 1
%gep_for = getelementptr inbounds float, float* %phi, i32 0
%gep_for_cast = bitcast float* %gep_for to i32*
%gep_for_cast_to_float = bitcast i32* %gep_for_cast to float*
%loop.cond = icmp ult i32 %i, 10
br i1 %loop.cond, label %for, label %end
end:
%phi_end = phi float* [ %gep_a_cast_to_float, %entry], [ %phi, %for ]
%phi_end.1 = bitcast float* %phi_end to i32*
%load = load i32, i32* %phi_end.1, align 4
ret i32 %load
}
declare %pair* @foo()
declare i32 @__gxx_personality_v0(...)