================
@@ -0,0 +1,120 @@
+# Key Instructions in Clang
+
+Key Instructions is an LLVM feature that reduces the jumpiness of optimized
code debug stepping. This document explains how Clang applies the necessary
metadata.
+
+## Implementation
+
+See the [LLVM docs](../../llvm/docs/KeyInstructionsDebugInfo.md) for general
info about the feature (and LLVM implementation details).
+
+Clang needs to annotate key instructions with the new metadata. Variable
assignments (stores, memory intrinsics), control flow (branches and their
conditions, some unconditional branches), and exception handling instructions
are annotated. Calls are ignored as they're unconditionally marked `is_stmt`.
This is achieved with a few simple constructs:
+
+Class `ApplyAtomGroup` - This is a scoped helper similar to
`ApplyDebugLocation` that creates a new source atom group which instructions
can be added to. It's used during CodeGen to declare that a new source atom has
started, e.g. in `CodeGenFunction::EmitBinaryOperatorLValue`.
+
+`CodeGenFunction::addInstToCurrentSourceAtom(llvm::Instruction
*KeyInstruction, llvm::Value *Backup)` adds an instruction (and a backup
instruction if non-null) to the current "atom group" defined with
`ApplyAtomGroup`. The Key Instruction gets rank 1, and backup instructions get
higher ranks (the function looks through casts, applying increasing rank as it
goes). There are a lot of sites in Clang that need to call this (mostly stores
and store-like instructions). FIXME?: Currently it's called at the CGBuilderTy
callsites; it could instead make sense to always call the function inside the
CGBuilderTy calls, with some calls opting out.
+
+`CodeGenFunction::addInstToNewSourceAtom(llvm::Instruction *KeyInstruction,
llvm::Value *Backup)` adds an instruction (and a backup instruction if
non-null) to a new "atom group". Currently mostly used in loop handling code.
+
+There are a couple of other helpers, including
`addRetToOverrideOrNewSourceAtom` used for `rets` which is covered in the
examples below.
+
+## Examples
+
+A simple example walk through:
+```
+void fun(int a) {
+ int b = a;
+}
+```
+
+There are two key instructions here, the assignment and the implicit return.
We want to emit metadata that looks like this:
+
+```
+define hidden void @_Z3funi(i32 noundef %a) #0 !dbg !11 {
+entry:
+ %a.addr = alloca i32, align 4
+ %b = alloca i32, align 4
+ store i32 %a, ptr %a.addr, align 4
+ %0 = load i32, ptr %a.addr, align 4, !dbg !DILocation(line: 2, scope: !11,
atomGroup: 1, atomRank: 2)
+ store i32 %0, ptr %b, align 4, !dbg !DILocation(line: 2, scope: !11,
atomGroup: 1, atomRank: 1)
+ ret void, !dbg !DILocation(line: 3, scope: !11,
atomGroup: 2, atomRank: 1)
+}
+```
+
+The store is the key instruction for the assignment (`atomGroup` 1). The
instruction corresponding to the final (and in this case only) RHS value, the
load from `%a.addr`, is a good backup location for `is_stmt` if the store gets
optimized away. It's part of the same source atom, but has lower `is_stmt`
precedence, so it gets a higher `atomRank`.
+
+This is all handled during CodeGen. The atom group is set here:
+```
+> clang::CodeGen::ApplyAtomGroup::ApplyAtomGroup(clang::CodeGen::CGDebugInfo
* DI) Line 187
+ clang::CodeGen::CodeGenFunction::EmitAutoVarInit(const
clang::CodeGen::CodeGenFunction::AutoVarEmission & emission) Line 1961
+ clang::CodeGen::CodeGenFunction::EmitAutoVarDecl(const clang::VarDecl & D)
Line 1361
+ clang::CodeGen::CodeGenFunction::EmitVarDecl(const clang::VarDecl & D) Line
219
+ clang::CodeGen::CodeGenFunction::EmitDecl(const clang::Decl & D) Line 164
+ clang::CodeGen::CodeGenFunction::EmitDeclStmt(const clang::DeclStmt & S)
Line 1611
+ clang::CodeGen::CodeGenFunction::EmitSimpleStmt(const clang::Stmt * S,
llvm::ArrayRef<clang::Attr const *> Attrs) Line 466
+ clang::CodeGen::CodeGenFunction::EmitStmt(const clang::Stmt * S,
llvm::ArrayRef<clang::Attr const *> Attrs) Line 72
+ clang::CodeGen::CodeGenFunction::EmitCompoundStmtWithoutScope(const
clang::CompoundStmt & S, bool GetLast, clang::CodeGen::AggValueSlot AggSlot)
Line 556+
+ clang::CodeGen::CodeGenFunction::EmitFunctionBody(const clang::Stmt * Body)
Line 1307
----------------
nikic wrote:
Uh, I don't think this listing (especially the line numbers!) is going to age
well..
https://github.com/llvm/llvm-project/pull/137991
_______________________________________________
cfe-commits mailing list
cfe-commits@lists.llvm.org
https://lists.llvm.org/cgi-bin/mailman/listinfo/cfe-commits
