From 6ae361ae458d4056d6c36fa42f1775c417457b10 Mon Sep 17 00:00:00 2001 From: Ian Lance Taylor Date: Fri, 23 Aug 2019 22:38:58 +0000 Subject: [PATCH] compiler: record pointer var values to remove write barriers Record when a local pointer variable is set to a value such that indirecting through the pointer does not require a write barrier. Use that to eliminate write barriers when indirecting through that local pointer variable. Only keep this information per-block, so it's not all that applicable. This reduces the number of write barriers generated when compiling the runtime package from 553 to 524. The point of this is to eliminate a bad write barrier in the bytes function in runtime/print.go. Mark that function nowritebarrier so that the problem does not recur. Reviewed-on: https://go-review.googlesource.com/c/gofrontend/+/191581 From-SVN: r274890 --- gcc/go/gofrontend/MERGE | 2 +- gcc/go/gofrontend/expressions.cc | 2 +- gcc/go/gofrontend/gogo.h | 9 ++- gcc/go/gofrontend/wb.cc | 115 ++++++++++++++++++++++++------- libgo/go/runtime/print.go | 1 + 5 files changed, 102 insertions(+), 27 deletions(-) diff --git a/gcc/go/gofrontend/MERGE b/gcc/go/gofrontend/MERGE index 5c0244662bc..73c75342fea 100644 --- a/gcc/go/gofrontend/MERGE +++ b/gcc/go/gofrontend/MERGE @@ -1,4 +1,4 @@ -82d27f0f140f33406cf59c0fb262f6dba3077f8e +c9ca1c6bf887c752cc75cf1ddaec8ddd1ec962d4 The first line of this file holds the git revision number of the last merge done from the gofrontend repository. diff --git a/gcc/go/gofrontend/expressions.cc b/gcc/go/gofrontend/expressions.cc index 378dab57a69..90a39a222b0 100644 --- a/gcc/go/gofrontend/expressions.cc +++ b/gcc/go/gofrontend/expressions.cc @@ -9039,7 +9039,7 @@ Builtin_call_expression::flatten_append(Gogo* gogo, Named_object* function, // directly before the write barrier pass. Statement* assign; if (assign_lhs != NULL - || !gogo->assign_needs_write_barrier(lhs)) + || !gogo->assign_needs_write_barrier(lhs, NULL)) assign = Statement::make_assignment(lhs, elem, loc); else { diff --git a/gcc/go/gofrontend/gogo.h b/gcc/go/gofrontend/gogo.h index 45217636116..087e89057ac 100644 --- a/gcc/go/gofrontend/gogo.h +++ b/gcc/go/gofrontend/gogo.h @@ -771,7 +771,14 @@ class Gogo // Return whether an assignment that sets LHS to RHS needs a write // barrier. bool - assign_needs_write_barrier(Expression* lhs); + assign_needs_write_barrier(Expression* lhs, + Unordered_set(const Named_object*)*); + + // Return whether EXPR is the address of a variable that can be set + // without a write barrier. That is, if this returns true, then an + // assignment to *EXPR does not require a write barrier. + bool + is_nonwb_pointer(Expression* expr, Unordered_set(const Named_object*)*); // Return an assignment that sets LHS to RHS using a write barrier. // This returns an if statement that checks whether write barriers diff --git a/gcc/go/gofrontend/wb.cc b/gcc/go/gofrontend/wb.cc index 501ad6a8ca2..41d8f94c074 100644 --- a/gcc/go/gofrontend/wb.cc +++ b/gcc/go/gofrontend/wb.cc @@ -402,13 +402,20 @@ class Write_barriers : public Traverse { public: Write_barriers(Gogo* gogo) - : Traverse(traverse_functions | traverse_variables | traverse_statements), - gogo_(gogo), function_(NULL), statements_added_() + : Traverse(traverse_functions + | traverse_blocks + | traverse_variables + | traverse_statements), + gogo_(gogo), function_(NULL), statements_added_(), + nonwb_pointers_() { } int function(Named_object*); + int + block(Block*); + int variable(Named_object*); @@ -422,6 +429,9 @@ class Write_barriers : public Traverse Function* function_; // Statements introduced. Statement_inserter::Statements statements_added_; + // Within a single block, pointer variables that point to values + // that do not need write barriers. + Unordered_set(const Named_object*) nonwb_pointers_; }; // Traverse a function. Just record it for later. @@ -439,6 +449,16 @@ Write_barriers::function(Named_object* no) return TRAVERSE_SKIP_COMPONENTS; } +// Traverse a block. Clear anything we know about local pointer +// variables. + +int +Write_barriers::block(Block*) +{ + this->nonwb_pointers_.clear(); + return TRAVERSE_CONTINUE; +} + // Insert write barriers for a global variable: ensure that variable // initialization is handled correctly. This is rarely needed, since // we currently don't enable background GC until after all global @@ -533,7 +553,16 @@ Write_barriers::statement(Block* block, size_t* pindex, Statement* s) // local variables get declaration statements, and local // variables on the stack do not require write barriers. if (!var->is_in_heap()) - break; + { + // If this is a pointer variable, and assigning through + // the initializer does not require a write barrier, + // record that fact. + if (var->type()->points_to() != NULL + && this->gogo_->is_nonwb_pointer(init, &this->nonwb_pointers_)) + this->nonwb_pointers_.insert(no); + + break; + } // Nothing to do if the variable does not contain any pointers. if (!var->type()->has_pointer()) @@ -578,15 +607,27 @@ Write_barriers::statement(Block* block, size_t* pindex, Statement* s) { Assignment_statement* as = s->assignment_statement(); - if (as->omit_write_barrier()) - break; - Expression* lhs = as->lhs(); Expression* rhs = as->rhs(); + // Keep track of variables whose values do not escape. + Var_expression* lhsve = lhs->var_expression(); + if (lhsve != NULL && lhsve->type()->points_to() != NULL) + { + Named_object* no = lhsve->named_object(); + if (this->gogo_->is_nonwb_pointer(rhs, &this->nonwb_pointers_)) + this->nonwb_pointers_.insert(no); + else + this->nonwb_pointers_.erase(no); + } + + if (as->omit_write_barrier()) + break; + // We may need to emit a write barrier for the assignment. - if (!this->gogo_->assign_needs_write_barrier(lhs)) + if (!this->gogo_->assign_needs_write_barrier(lhs, + &this->nonwb_pointers_)) break; // Change the assignment to use a write barrier. @@ -667,9 +708,13 @@ Gogo::write_barrier_variable() } // Return whether an assignment that sets LHS needs a write barrier. +// NONWB_POINTERS is a set of variables that point to values that do +// not need write barriers. bool -Gogo::assign_needs_write_barrier(Expression* lhs) +Gogo::assign_needs_write_barrier( + Expression* lhs, + Unordered_set(const Named_object*)* nonwb_pointers) { // Nothing to do if the variable does not contain any pointers. if (!lhs->type()->has_pointer()) @@ -738,22 +783,10 @@ Gogo::assign_needs_write_barrier(Expression* lhs) // Nothing to do for an assignment to *(convert(&x)) where // x is local variable or a temporary variable. Unary_expression* ue = lhs->unary_expression(); - if (ue != NULL && ue->op() == OPERATOR_MULT) - { - Expression* expr = ue->operand(); - while (true) - { - if (expr->conversion_expression() != NULL) - expr = expr->conversion_expression()->expr(); - else if (expr->unsafe_conversion_expression() != NULL) - expr = expr->unsafe_conversion_expression()->expr(); - else - break; - } - ue = expr->unary_expression(); - if (ue != NULL && ue->op() == OPERATOR_AND) - return this->assign_needs_write_barrier(ue->operand()); - } + if (ue != NULL + && ue->op() == OPERATOR_MULT + && this->is_nonwb_pointer(ue->operand(), nonwb_pointers)) + return false; // For a struct assignment, we don't need a write barrier if all the // pointer types can not be in the heap. @@ -784,6 +817,40 @@ Gogo::assign_needs_write_barrier(Expression* lhs) return true; } +// Return whether EXPR is the address of a variable that can be set +// without a write barrier. That is, if this returns true, then an +// assignment to *EXPR does not require a write barrier. +// NONWB_POINTERS is a set of variables that point to values that do +// not need write barriers. + +bool +Gogo::is_nonwb_pointer(Expression* expr, + Unordered_set(const Named_object*)* nonwb_pointers) +{ + while (true) + { + if (expr->conversion_expression() != NULL) + expr = expr->conversion_expression()->expr(); + else if (expr->unsafe_conversion_expression() != NULL) + expr = expr->unsafe_conversion_expression()->expr(); + else + break; + } + + Var_expression* ve = expr->var_expression(); + if (ve != NULL + && nonwb_pointers != NULL + && nonwb_pointers->find(ve->named_object()) != nonwb_pointers->end()) + return true; + + Unary_expression* ue = expr->unary_expression(); + if (ue == NULL || ue->op() != OPERATOR_AND) + return false; + if (this->assign_needs_write_barrier(ue->operand(), nonwb_pointers)) + return false; + return true; +} + // Return a statement that sets LHS to RHS using a write barrier. // ENCLOSING is the enclosing block. diff --git a/libgo/go/runtime/print.go b/libgo/go/runtime/print.go index 8c396442862..14d546d6d33 100644 --- a/libgo/go/runtime/print.go +++ b/libgo/go/runtime/print.go @@ -35,6 +35,7 @@ import ( // should use printhex instead of printuint (decimal). type hex uint64 +//go:nowritebarrier func bytes(s string) (ret []byte) { rp := (*slice)(unsafe.Pointer(&ret)) sp := stringStructOf(&s) -- 2.30.2