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codegen: optimize const fields of mutable objects (#53484)
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For example, we seek to eliminate the gc frame from this function, as
observed here:

```julia
julia> code_llvm((BitSet,), raw=true) do x; r = x.bits; GC.safepoint(); @inbounds r[1]; end
; Function Signature: var"https://github.com/JuliaLang/julia/issues/3"(Base.BitSet)
;  @ REPL[1]:1 within `https://github.com/JuliaLang/julia/issues/3`
define swiftcc i64 @"julia_#3_494"(ptr nonnull swiftself %pgcstack, ptr noundef nonnull align 8 dereferenceable(16) %"x::BitSet") #0 !dbg !5 {
top:
  call void @llvm.dbg.declare(metadata ptr %"x::BitSet", metadata !21, metadata !DIExpression()), !dbg !22
  %ptls_field = getelementptr inbounds ptr, ptr %pgcstack, i64 2
  %ptls_load = load ptr, ptr %ptls_field, align 8, !tbaa !23
  %0 = getelementptr inbounds ptr, ptr %ptls_load, i64 2
  %safepoint = load ptr, ptr %0, align 8, !tbaa !27
  fence syncscope("singlethread") seq_cst
  %1 = load volatile i64, ptr %safepoint, align 8, !dbg !22
  fence syncscope("singlethread") seq_cst
; ┌ @ Base.jl:49 within `getproperty`
   %"x::BitSet.bits" = load atomic ptr, ptr %"x::BitSet" unordered, align 8, !dbg !29, !tbaa !27, !alias.scope !33, !noalias !36, !nonnull !11, !dereferenceable !41, !align !42
; └
; ┌ @ gcutils.jl:253 within `safepoint`
   %ptls_load4 = load ptr, ptr %ptls_field, align 8, !dbg !43, !tbaa !23
   %2 = getelementptr inbounds ptr, ptr %ptls_load4, i64 2, !dbg !43
   %safepoint5 = load ptr, ptr %2, align 8, !dbg !43, !tbaa !27
   fence syncscope("singlethread") seq_cst, !dbg !43
   %3 = load volatile i64, ptr %safepoint5, align 8, !dbg !43
   fence syncscope("singlethread") seq_cst, !dbg !43
; └
; ┌ @ essentials.jl:892 within `getindex`
   %4 = load ptr, ptr %"x::BitSet.bits", align 8, !dbg !46, !tbaa !49, !alias.scope !52, !noalias !53
   %5 = load i64, ptr %4, align 8, !dbg !46, !tbaa !54, !alias.scope !57, !noalias !58
   ret i64 %5, !dbg !46
; └
}
```
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@vtjnash
vtjnash authored Mar 5, 2024
2 parents 3df016d + 50114d7 commit 58291db
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Showing 2 changed files with 130 additions and 42 deletions.
100 changes: 100 additions & 0 deletions src/cgutils.cpp
View file
Original file line number Diff line number Diff line change
Expand Up @@ -2644,6 +2644,104 @@ static jl_cgval_t emit_unionload(jl_codectx_t &ctx, Value *addr, Value *ptindex,
return mark_julia_slot(fsz > 0 ? addr : nullptr, jfty, tindex, tbaa);
}

static bool isTBAA(MDNode *TBAA, std::initializer_list<const char*> const strset)
{
if (!TBAA)
return false;
while (TBAA->getNumOperands() > 1) {
TBAA = cast<MDNode>(TBAA->getOperand(1).get());
auto str = cast<MDString>(TBAA->getOperand(0))->getString();
for (auto str2 : strset) {
if (str == str2) {
return true;
}
}
}
return false;
}

// Check if this is a load from an immutable value. The easiest
// way to do so is to look at the tbaa and see if it derives from
// jtbaa_immut.
static bool isLoadFromImmut(LoadInst *LI)
{
if (LI->getMetadata(LLVMContext::MD_invariant_load))
return true;
MDNode *TBAA = LI->getMetadata(LLVMContext::MD_tbaa);
if (isTBAA(TBAA, {"jtbaa_immut", "jtbaa_const", "jtbaa_datatype", "jtbaa_memoryptr", "jtbaa_memorylen", "jtbaa_memoryown"}))
return true;
return false;
}

static bool isConstGV(GlobalVariable *gv)
{
return gv->isConstant() || gv->getMetadata("julia.constgv");
}

// Check if this is can be traced through constant loads to an constant global
// or otherwise globally rooted value.
// Almost all `tbaa_const` loads satisfies this with the exception of
// task local constants which are constant as far as the code is concerned but aren't
// global constants. For task local constant `task_local` will be true when this function
// returns.
// Unlike this function in llvm-late-gc-lowering, we do not examine PhiNode, as those are not emitted yet
static bool isLoadFromConstGV(LoadInst *LI);
static bool isLoadFromConstGV(Value *v)
{
v = v->stripInBoundsOffsets();
if (auto LI = dyn_cast<LoadInst>(v))
return isLoadFromConstGV(LI);
if (auto gv = dyn_cast<GlobalVariable>(v))
return isConstGV(gv);
// null pointer
if (isa<ConstantData>(v))
return true;
// literal pointers
if (auto CE = dyn_cast<ConstantExpr>(v))
return (CE->getOpcode() == Instruction::IntToPtr &&
isa<ConstantData>(CE->getOperand(0)));
if (auto SL = dyn_cast<SelectInst>(v))
return (isLoadFromConstGV(SL->getTrueValue()) &&
isLoadFromConstGV(SL->getFalseValue()));
if (auto call = dyn_cast<CallInst>(v)) {
auto callee = call->getCalledFunction();
if (callee && callee->getName() == "julia.typeof") {
return true;
}
if (callee && callee->getName() == "julia.get_pgcstack") {
return true;
}
if (callee && callee->getName() == "julia.gc_loaded") {
return isLoadFromConstGV(call->getArgOperand(0)) &&
isLoadFromConstGV(call->getArgOperand(1));
}
}
if (isa<Argument>(v)) {
return true;
}
return false;
}

// The white list implemented here and above in `isLoadFromConstGV(Value*)` should
// cover all the cases we and LLVM generates.
static bool isLoadFromConstGV(LoadInst *LI)
{
// We only emit single slot GV in codegen
// but LLVM global merging can change the pointer operands to GEPs/bitcasts
auto load_base = LI->getPointerOperand()->stripInBoundsOffsets();
assert(load_base); // Static analyzer
auto gv = dyn_cast<GlobalVariable>(load_base);
if (isLoadFromImmut(LI)) {
if (gv)
return true;
return isLoadFromConstGV(load_base);
}
if (gv)
return isConstGV(gv);
return false;
}


static MDNode *best_field_tbaa(jl_codectx_t &ctx, const jl_cgval_t &strct, jl_datatype_t *jt, unsigned idx, size_t byte_offset)
{
auto tbaa = strct.tbaa;
Expand All @@ -2664,6 +2762,8 @@ static MDNode *best_field_tbaa(jl_codectx_t &ctx, const jl_cgval_t &strct, jl_da
return ctx.tbaa().tbaa_arraysize;
}
}
if (strct.V && jl_field_isconst(jt, idx) && isLoadFromConstGV(strct.V))
return ctx.tbaa().tbaa_const;
return tbaa;
}

Expand Down
72 changes: 30 additions & 42 deletions src/codegen.cpp
View file
Original file line number Diff line number Diff line change
Expand Up @@ -8267,18 +8267,18 @@ static jl_llvm_functions_t

// step 7. allocate local variables slots
// must be in the first basic block for the llvm mem2reg pass to work
auto allocate_local = [&](jl_varinfo_t &varinfo, jl_sym_t *s) {
auto allocate_local = [&ctx, &dbuilder, &debuginfo, topdebugloc, va, debug_enabled, M](jl_varinfo_t &varinfo, jl_sym_t *s, int i) {
jl_value_t *jt = varinfo.value.typ;
assert(!varinfo.boxroot); // variables shouldn't have memory locs already
if (varinfo.value.constant) {
// no need to explicitly load/store a constant/ghost value
alloc_def_flag(ctx, varinfo);
return;
}
else if (varinfo.isArgument && !(specsig && i == (size_t)ctx.vaSlot)) {
// if we can unbox it, just use the input pointer
if (i != (size_t)ctx.vaSlot && jl_is_concrete_immutable(jt))
return;
else if (varinfo.isArgument && (!va || ctx.vaSlot == -1 || i != ctx.vaSlot)) {
// just use the input pointer, if we have it
// (we will need to attach debuginfo later to it)
return;
}
else if (jl_is_uniontype(jt)) {
bool allunbox;
Expand All @@ -8289,6 +8289,7 @@ static jl_llvm_functions_t
varinfo.value = mark_julia_slot(lv, jt, NULL, ctx.tbaa().tbaa_stack);
varinfo.pTIndex = emit_static_alloca(ctx, getInt8Ty(ctx.builder.getContext()));
setName(ctx.emission_context, varinfo.pTIndex, "tindex");
// TODO: attach debug metadata to this variable
}
else if (allunbox) {
// all ghost values just need a selector allocated
Expand All @@ -8297,6 +8298,7 @@ static jl_llvm_functions_t
varinfo.pTIndex = lv;
varinfo.value.tbaa = NULL;
varinfo.value.isboxed = false;
// TODO: attach debug metadata to this variable
}
if (lv || allunbox)
alloc_def_flag(ctx, varinfo);
Expand All @@ -8323,29 +8325,21 @@ static jl_llvm_functions_t
}
return;
}
if (!varinfo.isArgument || // always need a slot if the variable is assigned
specsig || // for arguments, give them stack slots if they aren't in `argArray` (otherwise, will use that pointer)
(va && (int)i == ctx.vaSlot) || // or it's the va arg tuple
i == 0) { // or it is the first argument (which isn't in `argArray`)
AllocaInst *av = new AllocaInst(ctx.types().T_prjlvalue, M->getDataLayout().getAllocaAddrSpace(),
nullptr, Align(sizeof(jl_value_t*)), jl_symbol_name(s), /*InsertBefore*/ctx.topalloca);
StoreInst *SI = new StoreInst(Constant::getNullValue(ctx.types().T_prjlvalue), av, false, Align(sizeof(void*)));
SI->insertAfter(ctx.topalloca);
varinfo.boxroot = av;
if (debug_enabled && varinfo.dinfo) {
DIExpression *expr;
if ((Metadata*)varinfo.dinfo->getType() == debuginfo.jl_pvalue_dillvmt) {
expr = dbuilder.createExpression();
}
else {
SmallVector<uint64_t, 8> addr;
addr.push_back(llvm::dwarf::DW_OP_deref);
expr = dbuilder.createExpression(addr);
}
dbuilder.insertDeclare(av, varinfo.dinfo, expr,
topdebugloc,
ctx.builder.GetInsertBlock());
}
// otherwise give it a boxroot in this function
AllocaInst *av = new AllocaInst(ctx.types().T_prjlvalue, M->getDataLayout().getAllocaAddrSpace(),
nullptr, Align(sizeof(jl_value_t*)), jl_symbol_name(s), /*InsertBefore*/ctx.topalloca);
StoreInst *SI = new StoreInst(Constant::getNullValue(ctx.types().T_prjlvalue), av, false, Align(sizeof(void*)));
SI->insertAfter(ctx.topalloca);
varinfo.boxroot = av;
if (debug_enabled && varinfo.dinfo) {
SmallVector<uint64_t, 1> addr;
DIExpression *expr;
if ((Metadata*)varinfo.dinfo->getType() != debuginfo.jl_pvalue_dillvmt)
addr.push_back(llvm::dwarf::DW_OP_deref);
expr = dbuilder.createExpression(addr);
dbuilder.insertDeclare(av, varinfo.dinfo, expr,
topdebugloc,
ctx.builder.GetInsertBlock());
}
};

Expand All @@ -8359,7 +8353,7 @@ static jl_llvm_functions_t
varinfo.usedUndef = false;
continue;
}
allocate_local(varinfo, s);
allocate_local(varinfo, s, (int)i);
}

std::map<int, int> upsilon_to_phic;
Expand Down Expand Up @@ -8402,7 +8396,7 @@ static jl_llvm_functions_t
vi.used = true;
vi.isVolatile = true;
vi.value = mark_julia_type(ctx, (Value*)NULL, false, typ);
allocate_local(vi, jl_symbol("phic"));
allocate_local(vi, jl_symbol("phic"), -1);
}
}
}
Expand Down Expand Up @@ -8542,7 +8536,7 @@ static jl_llvm_functions_t
ctx.builder.CreateAlignedLoad(ctx.types().T_prjlvalue, argPtr, Align(sizeof(void*))),
false, vi.value.typ));
theArg = mark_julia_type(ctx, load, true, vi.value.typ);
if (debug_enabled && vi.dinfo && !vi.boxroot && !vi.value.V) {
if (debug_enabled && vi.dinfo && !vi.boxroot) {
SmallVector<uint64_t, 8> addr;
addr.push_back(llvm::dwarf::DW_OP_deref);
addr.push_back(llvm::dwarf::DW_OP_plus_uconst);
Expand All @@ -8561,21 +8555,15 @@ static jl_llvm_functions_t
assert(vi.value.V == NULL && "unexpected variable slot created for argument");
// keep track of original (possibly boxed) value to avoid re-boxing or moving
vi.value = theArg;
if (specsig && theArg.V && debug_enabled && vi.dinfo) {
SmallVector<uint64_t, 8> addr;
Value *parg;
if (debug_enabled && vi.dinfo && theArg.V) {
if (theArg.ispointer()) {
parg = theArg.V;
if ((Metadata*)vi.dinfo->getType() != debuginfo.jl_pvalue_dillvmt)
addr.push_back(llvm::dwarf::DW_OP_deref);
dbuilder.insertDeclare(theArg.V, vi.dinfo, dbuilder.createExpression(),
topdebugloc, ctx.builder.GetInsertBlock());
}
else {
parg = ctx.builder.CreateAlloca(theArg.V->getType(), NULL, jl_symbol_name(s));
ctx.builder.CreateStore(theArg.V, parg);
dbuilder.insertDbgValueIntrinsic(theArg.V, vi.dinfo, dbuilder.createExpression(),
topdebugloc, ctx.builder.GetInsertBlock());
}
dbuilder.insertDeclare(parg, vi.dinfo, dbuilder.createExpression(addr),
topdebugloc,
ctx.builder.GetInsertBlock());
}
}
else {
Expand Down

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