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Do not use optimized atomic libcalls for misaligned atomics.
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Summary:
The optimized (__atomic_foo_<n>) libcalls assume that the atomic object
is properly aligned, so should never be called on an underaligned
object.

This addresses one of several problems identified in PR38846.

Reviewers: jyknight, t.p.northover

Subscribers: jfb, cfe-commits

Differential Revision: https://reviews.llvm.org/D51817

llvm-svn: 341734
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@zygoloid
zygoloid committed Sep 7, 2018
1 parent 65f385d commit da3729d
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Showing 2 changed files with 47 additions and 14 deletions.
14 changes: 10 additions & 4 deletions clang/lib/CodeGen/CGAtomic.cpp
View file
Expand Up @@ -927,6 +927,15 @@ RValue CodeGenFunction::EmitAtomicExpr(AtomicExpr *E) {
UseOptimizedLibcall = true;
break;

case AtomicExpr::AO__atomic_load:
case AtomicExpr::AO__atomic_store:
case AtomicExpr::AO__atomic_exchange:
case AtomicExpr::AO__atomic_compare_exchange:
// Use the generic version if we don't know that the operand will be
// suitably aligned for the optimized version.
if (Misaligned)
break;
LLVM_FALLTHROUGH;
case AtomicExpr::AO__c11_atomic_load:
case AtomicExpr::AO__c11_atomic_store:
case AtomicExpr::AO__c11_atomic_exchange:
Expand All @@ -938,14 +947,11 @@ RValue CodeGenFunction::EmitAtomicExpr(AtomicExpr *E) {
case AtomicExpr::AO__opencl_atomic_compare_exchange_weak:
case AtomicExpr::AO__opencl_atomic_compare_exchange_strong:
case AtomicExpr::AO__atomic_load_n:
case AtomicExpr::AO__atomic_load:
case AtomicExpr::AO__atomic_store_n:
case AtomicExpr::AO__atomic_store:
case AtomicExpr::AO__atomic_exchange_n:
case AtomicExpr::AO__atomic_exchange:
case AtomicExpr::AO__atomic_compare_exchange_n:
case AtomicExpr::AO__atomic_compare_exchange:
// Only use optimized library calls for sizes for which they exist.
// FIXME: Size == 16 optimized library functions exist too.
if (Size == 1 || Size == 2 || Size == 4 || Size == 8)
UseOptimizedLibcall = true;
break;
Expand Down
47 changes: 37 additions & 10 deletions clang/test/CodeGen/atomic-ops.c
View file
Expand Up @@ -198,10 +198,12 @@ struct S implicit_load(_Atomic(struct S) *a) {
struct S fd1(struct S *a) {
// CHECK-LABEL: @fd1
// CHECK: [[RETVAL:%.*]] = alloca %struct.S, align 4
// CHECK: bitcast %struct.S* {{.*}} to i64*
// CHECK: [[A:%.*]] = bitcast %struct.S* {{.*}} to i64*
// CHECK: [[CAST:%.*]] = bitcast %struct.S* [[RETVAL]] to i64*
// CHECK: [[CALL:%.*]] = call i64 @__atomic_load_8(
// CHECK: store i64 [[CALL]], i64* [[CAST]], align 4
// CHECK: [[SRC:%.*]] = bitcast i64* [[A]] to i8*
// CHECK: [[DEST:%.*]] = bitcast i64* [[CAST]] to i8*
// CHECK: call void @__atomic_load(i32 8, i8* [[SRC]], i8* [[DEST]], i32 5)
// CHECK: ret
struct S ret;
__atomic_load(a, &ret, memory_order_seq_cst);
return ret;
Expand All @@ -218,8 +220,8 @@ void fd2(struct S *a, struct S *b) {
// CHECK-NEXT: [[COERCED_A_TMP:%.*]] = bitcast %struct.S* [[LOAD_A_PTR]] to i64*
// CHECK-NEXT: [[COERCED_B:%.*]] = bitcast %struct.S* [[LOAD_B_PTR]] to i64*
// CHECK-NEXT: [[COERCED_A:%.*]] = bitcast i64* [[COERCED_A_TMP]] to i8*
// CHECK-NEXT: [[LOAD_B:%.*]] = load i64, i64* [[COERCED_B]], align 4
// CHECK-NEXT: call void @__atomic_store_8(i8* [[COERCED_A]], i64 [[LOAD_B]],
// CHECK-NEXT: [[CAST_B:%.*]] = bitcast i64* [[COERCED_B]] to i8*
// CHECK-NEXT: call void @__atomic_store(i32 8, i8* [[COERCED_A]], i8* [[CAST_B]],
// CHECK-NEXT: ret void
__atomic_store(a, b, memory_order_seq_cst);
}
Expand All @@ -239,9 +241,9 @@ void fd3(struct S *a, struct S *b, struct S *c) {
// CHECK-NEXT: [[COERCED_B:%.*]] = bitcast %struct.S* [[LOAD_B_PTR]] to i64*
// CHECK-NEXT: [[COERCED_C:%.*]] = bitcast %struct.S* [[LOAD_C_PTR]] to i64*
// CHECK-NEXT: [[COERCED_A:%.*]] = bitcast i64* [[COERCED_A_TMP]] to i8*
// CHECK-NEXT: [[LOAD_B:%.*]] = load i64, i64* [[COERCED_B]], align 4
// CHECK-NEXT: [[CALL:%.*]] = call i64 @__atomic_exchange_8(i8* [[COERCED_A]], i64 [[LOAD_B]],
// CHECK-NEXT: store i64 [[CALL]], i64* [[COERCED_C]], align 4
// CHECK-NEXT: [[CAST_B:%.*]] = bitcast i64* [[COERCED_B]] to i8*
// CHECK-NEXT: [[CAST_C:%.*]] = bitcast i64* [[COERCED_C]] to i8*
// CHECK-NEXT: call void @__atomic_exchange(i32 8, i8* [[COERCED_A]], i8* [[CAST_B]], i8* [[CAST_C]],

__atomic_exchange(a, b, c, memory_order_seq_cst);
}
Expand All @@ -262,8 +264,8 @@ _Bool fd4(struct S *a, struct S *b, struct S *c) {
// CHECK-NEXT: [[COERCED_C:%.*]] = bitcast %struct.S* [[LOAD_C_PTR]] to i64*
// CHECK-NEXT: [[COERCED_A:%.*]] = bitcast i64* [[COERCED_A_TMP]] to i8*
// CHECK-NEXT: [[COERCED_B:%.*]] = bitcast i64* [[COERCED_B_TMP]] to i8*
// CHECK-NEXT: [[LOAD_C:%.*]] = load i64, i64* [[COERCED_C]], align 4
// CHECK-NEXT: [[CALL:%.*]] = call zeroext i1 @__atomic_compare_exchange_8(i8* [[COERCED_A]], i8* [[COERCED_B]], i64 [[LOAD_C]]
// CHECK-NEXT: [[CAST_C:%.*]] = bitcast i64* [[COERCED_C]] to i8*
// CHECK-NEXT: [[CALL:%.*]] = call zeroext i1 @__atomic_compare_exchange(i32 8, i8* [[COERCED_A]], i8* [[COERCED_B]], i8* [[CAST_C]],
// CHECK-NEXT: ret i1 [[CALL]]
return __atomic_compare_exchange(a, b, c, 1, 5, 5);
}
Expand Down Expand Up @@ -634,4 +636,29 @@ int PR17306_2(volatile int *i, int value) {
return __atomic_add_fetch(i, value, memory_order_seq_cst);
}

void test_underaligned() {
// CHECK-LABEL: @test_underaligned
struct Underaligned { char c[8]; } underaligned_a, underaligned_b, underaligned_c;

// CHECK: call void @__atomic_load(i32 8,
__atomic_load(&underaligned_a, &underaligned_b, memory_order_seq_cst);
// CHECK: call void @__atomic_store(i32 8,
__atomic_store(&underaligned_a, &underaligned_b, memory_order_seq_cst);
// CHECK: call void @__atomic_exchange(i32 8,
__atomic_exchange(&underaligned_a, &underaligned_b, &underaligned_c, memory_order_seq_cst);
// CHECK: call {{.*}} @__atomic_compare_exchange(i32 8,
__atomic_compare_exchange(&underaligned_a, &underaligned_b, &underaligned_c, 1, memory_order_seq_cst, memory_order_seq_cst);

__attribute__((aligned)) struct Underaligned aligned_a, aligned_b, aligned_c;

// CHECK: load atomic
__atomic_load(&aligned_a, &aligned_b, memory_order_seq_cst);
// CHECK: store atomic
__atomic_store(&aligned_a, &aligned_b, memory_order_seq_cst);
// CHECK: atomicrmw xchg
__atomic_exchange(&aligned_a, &aligned_b, &aligned_c, memory_order_seq_cst);
// CHECK: cmpxchg weak
__atomic_compare_exchange(&aligned_a, &aligned_b, &aligned_c, 1, memory_order_seq_cst, memory_order_seq_cst);
}

#endif

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