https://github.com/fhahn created
https://github.com/llvm/llvm-project/pull/121789
This patch adds a new __builtin_assume_dereferenceable to encode
dereferenceability of a pointer using llvm.assume with an operand bundle.
For now the builtin only accepts constant sizes, I am planning to drop this
restriction in a follow-up change.
This can be used to better optimize cases where a pointer is known to be
dereferenceable, e.g. unconditionally loading from p2 when vectorizing the loop.
int *get_ptr();
void foo(int* src, int x) {
int *p2 = get_ptr();
__builtin_assume_aligned(p2, 4);
__builtin_assume_dereferenceable(p2, 4000);
for (unsigned I = 0; I != 1000; ++I) {
int x = src[I];
if (x == 0)
x = p2[I];
src[I] = x;
}
}
>From 9facf13c1678dac6168a0d1c5103b513a2a71222 Mon Sep 17 00:00:00 2001
From: Florian Hahn <f...@fhahn.com>
Date: Mon, 6 Jan 2025 13:39:55 +0000
Subject: [PATCH] [Clang] Add __builtin_assume_dereferenceable to encode deref
assumption.
This patch adds a new __builtin_assume_dereferenceable to encode
dereferenceability of a pointer using llvm.assume with an operand
bundle.
For now the builtin only accepts constant sizes, I am planning to drop
this restriction in a follow-up change.
This can be used to better optimize cases where a pointer is known to be
dereferenceable, e.g. unconditionally loading from p2 when vectorizing the loop.
int *get_ptr();
void foo(int* src, int x) {
int *p2 = get_ptr();
__builtin_assume_aligned(p2, 4);
__builtin_assume_dereferenceable(p2, 4000);
for (unsigned I = 0; I != 1000; ++I) {
int x = src[I];
if (x == 0)
x = p2[I];
src[I] = x;
}
}
---
clang/docs/LanguageExtensions.rst | 35 ++++++++++++++++
clang/include/clang/Basic/Builtins.td | 6 +++
clang/lib/CodeGen/CGBuiltin.cpp | 10 +++++
.../CodeGen/builtin-assume-dereferenceable.c | 34 +++++++++++++++
.../Sema/builtin-assume-dereferenceable.c | 41 +++++++++++++++++++
llvm/include/llvm/IR/IRBuilder.h | 5 +++
llvm/lib/IR/IRBuilder.cpp | 10 +++++
7 files changed, 141 insertions(+)
create mode 100644 clang/test/CodeGen/builtin-assume-dereferenceable.c
create mode 100644 clang/test/Sema/builtin-assume-dereferenceable.c
diff --git a/clang/docs/LanguageExtensions.rst
b/clang/docs/LanguageExtensions.rst
index e020710c7aa4f5..1a96647ae765e2 100644
--- a/clang/docs/LanguageExtensions.rst
+++ b/clang/docs/LanguageExtensions.rst
@@ -2761,6 +2761,41 @@ etc.).
Query for this feature with
``__has_builtin(__builtin_assume_separate_storage)``.
+``__builtin_assume_dereferenceable``
+-------------------------------------
+
+``__builtin_assume_derefernceable`` is used to provide the optimizer with the
+knowledge that the pointer argument P is dereferenceable up to the specified
+number of bytes.
+
+**Syntax**:
+
+.. code-block:: c++
+
+ __builtin_assume_dereferenceable(const void *, size_t)
+
+**Example of Use**:
+
+.. code-block:: c++
+
+ int foo(int *x, int y) {
+ __builtin_assume_dereferenceable(x, 4);
+ int z = 0;
+ if (y == 1) {
+ // The optimizer may execute the load of x unconditionally.
+ z = *x;
+ }
+ return z;
+ }
+
+**Description**:
+
+The arguments to this function provide a start pointer ``P`` and a size ``S``.
+``P`` must be non-null and ``S`` at least 1. The optimizer may assume that
+``S`` bytes are dereferenceable starting at ``P``.
+
+Query for this feature with
``__has_builtin(__builtin_assume_dereferenceable)``.
+
``__builtin_offsetof``
----------------------
diff --git a/clang/include/clang/Basic/Builtins.td
b/clang/include/clang/Basic/Builtins.td
index 468c16050e2bf0..e9d8c6f621afa9 100644
--- a/clang/include/clang/Basic/Builtins.td
+++ b/clang/include/clang/Basic/Builtins.td
@@ -839,6 +839,12 @@ def BuiltinAssumeAligned : Builtin {
let Prototype = "void*(void const*, size_t, ...)";
}
+def BuiltinAssumeDereferenceable : Builtin {
+ let Spellings = ["__builtin_assume_dereferenceable"];
+ let Attributes = [NoThrow, Const, Constexpr];
+ let Prototype = "void(void const*, _Constant size_t)";
+}
+
def BuiltinFree : Builtin {
let Spellings = ["__builtin_free"];
let Attributes = [FunctionWithBuiltinPrefix, NoThrow];
diff --git a/clang/lib/CodeGen/CGBuiltin.cpp b/clang/lib/CodeGen/CGBuiltin.cpp
index c419fb0cc055e0..0743d4d5eef49e 100644
--- a/clang/lib/CodeGen/CGBuiltin.cpp
+++ b/clang/lib/CodeGen/CGBuiltin.cpp
@@ -3726,6 +3726,16 @@ RValue CodeGenFunction::EmitBuiltinExpr(const GlobalDecl
GD, unsigned BuiltinID,
AlignmentCI, OffsetValue);
return RValue::get(PtrValue);
}
+ case Builtin::BI__builtin_assume_dereferenceable: {
+ const Expr *Ptr = E->getArg(0);
+ Value *PtrValue = EmitScalarExpr(Ptr);
+ Value *SizeValue = EmitScalarExpr(E->getArg(1));
+ if (SizeValue->getType() != IntPtrTy)
+ SizeValue =
+ Builder.CreateIntCast(SizeValue, IntPtrTy, false, "casted.size");
+ Builder.CreateDereferenceableAssumption(PtrValue, SizeValue);
+ return RValue::get(nullptr);
+ }
case Builtin::BI__assume:
case Builtin::BI__builtin_assume: {
if (E->getArg(0)->HasSideEffects(getContext()))
diff --git a/clang/test/CodeGen/builtin-assume-dereferenceable.c
b/clang/test/CodeGen/builtin-assume-dereferenceable.c
new file mode 100644
index 00000000000000..cadffd4a84c264
--- /dev/null
+++ b/clang/test/CodeGen/builtin-assume-dereferenceable.c
@@ -0,0 +1,34 @@
+// NOTE: Assertions have been autogenerated by utils/update_cc_test_checks.py
+// RUN: %clang_cc1 -triple x86_64-unknown-unknown -emit-llvm -o - %s |
FileCheck %s
+
+// CHECK-LABEL: @test1(
+// CHECK-NEXT: entry:
+// CHECK-NEXT: [[A_ADDR:%.*]] = alloca ptr, align 8
+// CHECK-NEXT: store ptr [[A:%.*]], ptr [[A_ADDR]], align 8
+// CHECK-NEXT: [[TMP0:%.*]] = load ptr, ptr [[A_ADDR]], align 8
+// CHECK-NEXT: call void @llvm.assume(i1 true) [ "dereferenceable"(ptr
[[TMP0]], i64 10) ]
+// CHECK-NEXT: [[TMP1:%.*]] = load ptr, ptr [[A_ADDR]], align 8
+// CHECK-NEXT: [[ARRAYIDX:%.*]] = getelementptr inbounds i32, ptr [[TMP1]],
i64 0
+// CHECK-NEXT: [[TMP2:%.*]] = load i32, ptr [[ARRAYIDX]], align 4
+// CHECK-NEXT: ret i32 [[TMP2]]
+//
+int test1(int *a) {
+ __builtin_assume_dereferenceable(a, 10);
+ return a[0];
+}
+
+// CHECK-LABEL: @test2(
+// CHECK-NEXT: entry:
+// CHECK-NEXT: [[A_ADDR:%.*]] = alloca ptr, align 8
+// CHECK-NEXT: store ptr [[A:%.*]], ptr [[A_ADDR]], align 8
+// CHECK-NEXT: [[TMP0:%.*]] = load ptr, ptr [[A_ADDR]], align 8
+// CHECK-NEXT: call void @llvm.assume(i1 true) [ "dereferenceable"(ptr
[[TMP0]], i64 32) ]
+// CHECK-NEXT: [[TMP1:%.*]] = load ptr, ptr [[A_ADDR]], align 8
+// CHECK-NEXT: [[ARRAYIDX:%.*]] = getelementptr inbounds i32, ptr [[TMP1]],
i64 0
+// CHECK-NEXT: [[TMP2:%.*]] = load i32, ptr [[ARRAYIDX]], align 4
+// CHECK-NEXT: ret i32 [[TMP2]]
+//
+int test2(int *a) {
+ __builtin_assume_dereferenceable(a, 32ull);
+ return a[0];
+}
diff --git a/clang/test/Sema/builtin-assume-dereferenceable.c
b/clang/test/Sema/builtin-assume-dereferenceable.c
new file mode 100644
index 00000000000000..07b23022043cd7
--- /dev/null
+++ b/clang/test/Sema/builtin-assume-dereferenceable.c
@@ -0,0 +1,41 @@
+// RUN: %clang_cc1 -DSIZE_T_64 -fsyntax-only -Wno-strict-prototypes -triple
x86_64-linux -verify %s
+
+int test1(int *a) {
+ __builtin_assume_dereferenceable(a, 32);
+ return a[0];
+}
+
+int test2(int *a) {
+ __builtin_assume_dereferenceable(a, 32ull);
+ return a[0];
+}
+
+int test3(int *a) {
+ __builtin_assume_dereferenceable(a, 32u);
+ return a[0];
+}
+
+int test4(int *a, unsigned size) {
+ a = __builtin_assume_dereferenceable(a, size); // expected-error {{argument
to '__builtin_assume_dereferenceable' must be a constant integer}}
+ return a[0];
+}
+
+int test5(int *a, unsigned long long size) {
+ a = __builtin_assume_dereferenceable(a, size); // expected-error {{argument
to '__builtin_assume_dereferenceable' must be a constant integer}}
+ return a[0];
+}
+
+int test6(float a) {
+ __builtin_assume_dereferenceable(a, 2); // expected-error {{passing 'float'
to parameter of incompatible type 'const void *'}}
+ return 0;;
+}
+
+int test7(int *a) {
+ __builtin_assume_dereferenceable(a, 32, 1); // expected-error {{too many
arguments to function call, expected 2, have 3}}
+ return a[0];
+}
+
+int test8(int *a) {
+ __builtin_assume_dereferenceable(a); // expected-error {{too few arguments
to function call, expected 2, have 1}}
+ return a[0];
+}
diff --git a/llvm/include/llvm/IR/IRBuilder.h b/llvm/include/llvm/IR/IRBuilder.h
index b73309175f20d1..4da303d28e3d7b 100644
--- a/llvm/include/llvm/IR/IRBuilder.h
+++ b/llvm/include/llvm/IR/IRBuilder.h
@@ -2678,6 +2678,11 @@ class IRBuilderBase {
CallInst *CreateAlignmentAssumption(const DataLayout &DL, Value *PtrValue,
Value *Alignment,
Value *OffsetValue = nullptr);
+
+ /// Create an assume intrinsic call that represents an dereferencable
+ /// assumption on the provided pointer.
+ ///
+ CallInst *CreateDereferenceableAssumption(Value *PtrValue, Value *SizeValue);
};
/// This provides a uniform API for creating instructions and inserting
diff --git a/llvm/lib/IR/IRBuilder.cpp b/llvm/lib/IR/IRBuilder.cpp
index 27b499e42a4e4c..87636a33ee044b 100644
--- a/llvm/lib/IR/IRBuilder.cpp
+++ b/llvm/lib/IR/IRBuilder.cpp
@@ -1274,6 +1274,16 @@ CallInst *IRBuilderBase::CreateAlignmentAssumption(const
DataLayout &DL,
return CreateAlignmentAssumptionHelper(DL, PtrValue, Alignment, OffsetValue);
}
+CallInst *IRBuilderBase::CreateDereferenceableAssumption(Value *PtrValue,
+ Value *SizeValue) {
+ assert(isa<PointerType>(PtrValue->getType()) &&
+ "trying to create an deferenceable assumption on a non-pointer?");
+ SmallVector<Value *, 4> Vals({PtrValue, SizeValue});
+ OperandBundleDefT<Value *> DereferenceableOpB("dereferenceable", Vals);
+ return CreateAssumption(ConstantInt::getTrue(getContext()),
+ {DereferenceableOpB});
+}
+
IRBuilderDefaultInserter::~IRBuilderDefaultInserter() = default;
IRBuilderCallbackInserter::~IRBuilderCallbackInserter() = default;
IRBuilderFolder::~IRBuilderFolder() = default;
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