llvmbot wrote:
@llvm/pr-subscribers-backend-x86
Author: Anton Bikineev (AntonBikineev)
Changes
The existing attribute works only for loop headers. This can unfortunately be
quite limiting, especially as a protection against the "Jump Conditional Code
Erratum" [0] (for example, if there is an unaligned check in a hot loop). The
command line option -malign-branch-boundary can help with that, but it's too
coarse-grained and can significantly increase the binary size.
This change introduces a per-function [[clang::code_align(N)]] attribute that
aligns all the basic blocks of a function by the given N.
[0]
https://www.intel.com/content/dam/support/us/en/documents/processors/mitigations-jump-conditional-code-erratum.pdf
---
Full diff: https://github.com/llvm/llvm-project/pull/80765.diff
8 Files Affected:
- (modified) clang/include/clang/Basic/Attr.td (+4-3)
- (modified) clang/include/clang/Basic/AttrDocs.td (+11-5)
- (modified) clang/lib/CodeGen/CodeGenModule.cpp (+6)
- (modified) clang/lib/Sema/SemaDeclAttr.cpp (+10)
- (added) clang/test/CodeGen/code_align_function.c (+42)
- (modified) clang/test/Sema/code_align.c (+12-2)
- (modified) llvm/lib/CodeGen/MachineBlockPlacement.cpp (+21-6)
- (added) llvm/test/CodeGen/X86/code-align-basic-blocks.ll (+29)
``diff
diff --git a/clang/include/clang/Basic/Attr.td
b/clang/include/clang/Basic/Attr.td
index b2d5309e142c1..abce685e9f7a6 100644
--- a/clang/include/clang/Basic/Attr.td
+++ b/clang/include/clang/Basic/Attr.td
@@ -4435,10 +4435,11 @@ def PreferredType: InheritableAttr {
let Documentation = [PreferredTypeDocumentation];
}
-def CodeAlign: StmtAttr {
+def CodeAlign : InheritableAttr {
let Spellings = [Clang<"code_align">];
- let Subjects = SubjectList<[ForStmt, CXXForRangeStmt, WhileStmt, DoStmt],
- ErrorDiag, "'for', 'while', and 'do'
statements">;
+ let Subjects =
+ SubjectList<[ForStmt, CXXForRangeStmt, WhileStmt, DoStmt, Function],
+ ErrorDiag, "'for', 'while', 'do' statements and functions">;
let Args = [ExprArgument<"Alignment">];
let Documentation = [CodeAlignAttrDocs];
let AdditionalMembers = [{
diff --git a/clang/include/clang/Basic/AttrDocs.td
b/clang/include/clang/Basic/AttrDocs.td
index 041786f37fb8a..57b63469f1573 100644
--- a/clang/include/clang/Basic/AttrDocs.td
+++ b/clang/include/clang/Basic/AttrDocs.td
@@ -7704,11 +7704,12 @@ def CodeAlignAttrDocs : Documentation {
let Category = DocCatVariable;
let Heading = "clang::code_align";
let Content = [{
-The ``clang::code_align(N)`` attribute applies to a loop and specifies the byte
-alignment for a loop. The attribute accepts a positive integer constant
-initialization expression indicating the number of bytes for the minimum
-alignment boundary. Its value must be a power of 2, between 1 and 4096
-(inclusive).
+The ``clang::code_align(N)`` attribute applies to a loop or a function. When
+applied to a loop it specifies the byte alignment for the loop header. When
+applied to a function it aligns all the basic blocks of the function. The
+attribute accepts a positive integer constant initialization expression
+indicating the number of bytes for the minimum alignment boundary. Its value
+must be a power of 2, between 1 and 4096 (inclusive).
.. code-block:: c++
@@ -7738,6 +7739,11 @@ alignment boundary. Its value must be a power of 2,
between 1 and 4096
[[clang::code_align(A)]] for(;;) { }
}
+ [[clang:code_align(16)]] int foo(bool b) {
+if (b) return 2;
+return 3;
+ }
+
}];
}
diff --git a/clang/lib/CodeGen/CodeGenModule.cpp
b/clang/lib/CodeGen/CodeGenModule.cpp
index 36b63d78b06f8..87a822e91fff6 100644
--- a/clang/lib/CodeGen/CodeGenModule.cpp
+++ b/clang/lib/CodeGen/CodeGenModule.cpp
@@ -2515,6 +2515,12 @@ void
CodeGenModule::SetLLVMFunctionAttributesForDefinition(const Decl *D,
F->setAlignment(std::max(llvm::Align(2), F->getAlign().valueOrOne()));
}
+ if (const auto *CodeAlign = D->getAttr()) {
+const auto *CE = cast(CodeAlign->getAlignment());
+std::string ArgValStr = llvm::toString(CE->getResultAsAPSInt(), 10);
+F->addFnAttr("align-basic-blocks", ArgValStr);
+ }
+
// In the cross-dso CFI mode with canonical jump tables, we want !type
// attributes on definitions only.
if (CodeGenOpts.SanitizeCfiCrossDso &&
diff --git a/clang/lib/Sema/SemaDeclAttr.cpp b/clang/lib/Sema/SemaDeclAttr.cpp
index d785714c4d811..40412801b632a 100644
--- a/clang/lib/Sema/SemaDeclAttr.cpp
+++ b/clang/lib/Sema/SemaDeclAttr.cpp
@@ -9036,6 +9036,12 @@ static void handleArmNewAttr(Sema &S, Decl *D, const
ParsedAttr &AL) {
ArmNewAttr(S.Context, AL, NewState.data(), NewState.size()));
}
+static void handleCodeAlignAttr(Sema &S, Decl *D, const ParsedAttr &A) {
+ Expr *E = A.getArgAsExpr(0);
+ if (Attr *CodeAlignAttr = S.BuildCodeAlignAttr(A, E))
+D->addAttr(CodeAlignAttr);
+}
+
/// ProcessDeclAttribute - Apply the specific
