On Sat, Oct 14, 2023 at 01:13:22AM -0400, Jason Merrill wrote:
> On 10/13/23 14:53, Marek Polacek wrote:
> > On Thu, Oct 12, 2023 at 09:41:43PM -0400, Jason Merrill wrote:
> > > On 10/12/23 17:04, Marek Polacek wrote:
> > > > Bootstrapped/regtested on x86_64-pc-linux-gnu, ok for trunk?
> > > >
> > > > -- >8 --
> > > > My recent patch introducing cp_fold_immediate_r caused exponential
> > > > compile time with nested COND_EXPRs. The problem is that the COND_EXPR
> > > > case recursively walks the arms of a COND_EXPR, but after processing
> > > > both arms it doesn't end the walk; it proceeds to walk the
> > > > sub-expressions of the outermost COND_EXPR, triggering again walking
> > > > the arms of the nested COND_EXPR, and so on. This patch brings the
> > > > compile time down to about 0m0.033s.
> > > >
> > > > I've added some debug prints to make sure that the rest of cp_fold_r
> > > > is still performed as before.
> > > >
> > > > PR c++/111660
> > > >
> > > > gcc/cp/ChangeLog:
> > > >
> > > > * cp-gimplify.cc (cp_fold_immediate_r) <case COND_EXPR>:
> > > > Return
> > > > integer_zero_node instead of break;.
> > > > (cp_fold_immediate): Return true if cp_fold_immediate_r
> > > > returned
> > > > error_mark_node.
> > > >
> > > > gcc/testsuite/ChangeLog:
> > > >
> > > > * g++.dg/cpp0x/hog1.C: New test.
> > > > ---
> > > > gcc/cp/cp-gimplify.cc | 9 ++--
> > > > gcc/testsuite/g++.dg/cpp0x/hog1.C | 77
> > > > +++++++++++++++++++++++++++++++
> > > > 2 files changed, 82 insertions(+), 4 deletions(-)
> > > > create mode 100644 gcc/testsuite/g++.dg/cpp0x/hog1.C
> > > >
> > > > diff --git a/gcc/cp/cp-gimplify.cc b/gcc/cp/cp-gimplify.cc
> > > > index bdf6e5f98ff..ca622ca169a 100644
> > > > --- a/gcc/cp/cp-gimplify.cc
> > > > +++ b/gcc/cp/cp-gimplify.cc
> > > > @@ -1063,16 +1063,16 @@ cp_fold_immediate_r (tree *stmt_p, int
> > > > *walk_subtrees, void *data_)
> > > > break;
> > > > if (TREE_OPERAND (stmt, 1)
> > > > && cp_walk_tree (&TREE_OPERAND (stmt, 1),
> > > > cp_fold_immediate_r, data,
> > > > - nullptr))
> > > > + nullptr) == error_mark_node)
> > > > return error_mark_node;
> > > > if (TREE_OPERAND (stmt, 2)
> > > > && cp_walk_tree (&TREE_OPERAND (stmt, 2),
> > > > cp_fold_immediate_r, data,
> > > > - nullptr))
> > > > + nullptr) == error_mark_node)
> > > > return error_mark_node;
> > > > /* We're done here. Don't clear *walk_subtrees here though:
> > > > we're called
> > > > from cp_fold_r and we must let it recurse on the expression
> > > > with
> > > > cp_fold. */
> > > > - break;
> > > > + return integer_zero_node;
> > >
> > > I'm concerned this will end up missing something like
> > >
> > > 1 ? 1 : ((1 ? 1 : 1), immediate())
> > >
> > > as the integer_zero_node from the inner ?: will prevent walk_tree from
> > > looking any farther.
> >
> > You are right. The line above works as expected, but
> >
> > 1 ? 1 : ((1 ? 1 : id (42)), id (i));
> >
> > shows the problem (when the expression isn't used as an initializer).
> >
> > > Maybe we want to handle COND_EXPR in cp_fold_r instead of here?
> >
> > I hope this version is better.
> >
> > Bootstrapped/regtested on x86_64-pc-linux-gnu, ok for trunk?
> >
> > -- >8 --
> > My recent patch introducing cp_fold_immediate_r caused exponential
> > compile time with nested COND_EXPRs. The problem is that the COND_EXPR
> > case recursively walks the arms of a COND_EXPR, but after processing
> > both arms it doesn't end the walk; it proceeds to walk the
> > sub-expressions of the outermost COND_EXPR, triggering again walking
> > the arms of the nested COND_EXPR, and so on. This patch brings the
> > compile time down to about 0m0.033s.
>
> Is this number still accurate for this version?
It is. I ran time(1) a few more times and the results were 0m0.033s - 0m0.035s.
That said, ...
> This change seems algorithmically better than the current code, but still
> problematic: if we have nested COND_EXPR A/B/C/D/E, it looks like we will
> end up cp_fold_immediate_r walking the arms of E five times, once for each
> COND_EXPR.
...this is accurate. I should have addressed the redundant folding in v2
even though the compilation is pretty much immediate.
> What I was thinking by handling COND_EXPR in cp_fold_r was to cp_fold_r walk
> its subtrees (or cp_fold_immediate_r if it's clear from op0 that the branch
> isn't taken) so we can clear *walk_subtrees and we don't fold_imm walk a
> node more than once.
I agree I should do better here. How's this, then? I've added
debug_generic_expr to cp_fold_immediate_r to see if it gets the same
expr multiple times and it doesn't seem to.
Bootstrapped/regtested on x86_64-pc-linux-gnu, ok for trunk?
-- >8 --
My recent patch introducing cp_fold_immediate_r caused exponential
compile time with nested COND_EXPRs. The problem is that the COND_EXPR
case recursively walks the arms of a COND_EXPR, but after processing
both arms it doesn't end the walk; it proceeds to walk the
sub-expressions of the outermost COND_EXPR, triggering again walking
the arms of the nested COND_EXPR, and so on. This patch brings the
compile time down to about 0m0.030s.
The ff_fold_immediate flag is unused after this patch but since I'm
using it in the P2564 patch, I'm not removing it now. Maybe at_eof
can be used instead and then we can remove ff_fold_immediate.
PR c++/111660
gcc/cp/ChangeLog:
* cp-gimplify.cc (cp_fold_immediate_r) <case COND_EXPR>: Don't
handle it here.
(cp_fold_r): Handle COND_EXPR here.
gcc/testsuite/ChangeLog:
* g++.dg/cpp0x/hog1.C: New test.
* g++.dg/cpp2a/consteval36.C: New test.
---
gcc/cp/cp-gimplify.cc | 52 +++++++++-------
gcc/testsuite/g++.dg/cpp0x/hog1.C | 77 ++++++++++++++++++++++++
gcc/testsuite/g++.dg/cpp2a/consteval36.C | 22 +++++++
3 files changed, 128 insertions(+), 23 deletions(-)
create mode 100644 gcc/testsuite/g++.dg/cpp0x/hog1.C
create mode 100644 gcc/testsuite/g++.dg/cpp2a/consteval36.C
diff --git a/gcc/cp/cp-gimplify.cc b/gcc/cp/cp-gimplify.cc
index bdf6e5f98ff..a282c3930a3 100644
--- a/gcc/cp/cp-gimplify.cc
+++ b/gcc/cp/cp-gimplify.cc
@@ -1052,27 +1052,6 @@ cp_fold_immediate_r (tree *stmt_p, int *walk_subtrees,
void *data_)
switch (TREE_CODE (stmt))
{
- /* Unfortunately we must handle code like
- false ? bar () : 42
- where we have to check bar too. The cp_fold call in cp_fold_r could
- fold the ?: into a constant before we see it here. */
- case COND_EXPR:
- /* If we are called from cp_fold_immediate, we don't need to worry about
- cp_fold folding away the COND_EXPR. */
- if (data->flags & ff_fold_immediate)
- break;
- if (TREE_OPERAND (stmt, 1)
- && cp_walk_tree (&TREE_OPERAND (stmt, 1), cp_fold_immediate_r, data,
- nullptr))
- return error_mark_node;
- if (TREE_OPERAND (stmt, 2)
- && cp_walk_tree (&TREE_OPERAND (stmt, 2), cp_fold_immediate_r, data,
- nullptr))
- return error_mark_node;
- /* We're done here. Don't clear *walk_subtrees here though: we're called
- from cp_fold_r and we must let it recurse on the expression with
- cp_fold. */
- break;
case PTRMEM_CST:
if (TREE_CODE (PTRMEM_CST_MEMBER (stmt)) == FUNCTION_DECL
&& DECL_IMMEDIATE_FUNCTION_P (PTRMEM_CST_MEMBER (stmt)))
@@ -1162,8 +1141,35 @@ cp_fold_r (tree *stmt_p, int *walk_subtrees, void *data_)
tree stmt = *stmt_p;
enum tree_code code = TREE_CODE (stmt);
- if (cxx_dialect > cxx17)
- cp_fold_immediate_r (stmt_p, walk_subtrees, data);
+ if (cxx_dialect >= cxx20)
+ {
+ /* Unfortunately we must handle code like
+ false ? bar () : 42
+ where we have to check bar too. The cp_fold call below could
+ fold the ?: into a constant before we've checked it. */
+ if (code == COND_EXPR)
+ {
+ auto then_fn = cp_fold_r, else_fn = cp_fold_r;
+ /* See if we can figure out if either of the branches is dead. If it
+ is, we don't need to do everything that cp_fold_r does. */
+ tree cond = maybe_constant_value (TREE_OPERAND (stmt, 0));
+ if (integer_zerop (cond))
+ then_fn = cp_fold_immediate_r;
+ else if (TREE_CODE (cond) == INTEGER_CST)
+ else_fn = cp_fold_immediate_r;
+
+ cp_walk_tree (&TREE_OPERAND (stmt, 0), cp_fold_r, data, nullptr);
+ if (TREE_OPERAND (stmt, 1))
+ cp_walk_tree (&TREE_OPERAND (stmt, 1), then_fn, data,
+ nullptr);
+ if (TREE_OPERAND (stmt, 2))
+ cp_walk_tree (&TREE_OPERAND (stmt, 2), else_fn, data,
+ nullptr);
+ *walk_subtrees = 0;
+ /* Don't return yet, still need the cp_fold below. */
+ }
+ cp_fold_immediate_r (stmt_p, walk_subtrees, data);
+ }
*stmt_p = stmt = cp_fold (*stmt_p, data->flags);
diff --git a/gcc/testsuite/g++.dg/cpp0x/hog1.C
b/gcc/testsuite/g++.dg/cpp0x/hog1.C
new file mode 100644
index 00000000000..105a2e912c4
--- /dev/null
+++ b/gcc/testsuite/g++.dg/cpp0x/hog1.C
@@ -0,0 +1,77 @@
+// PR c++/111660
+// { dg-do compile { target c++11 } }
+
+enum Value {
+ LPAREN,
+ RPAREN,
+ LBRACE,
+ RBRACE,
+ LBRACK,
+ RBRACK,
+ CONDITIONAL,
+ COLON,
+ SEMICOLON,
+ COMMA,
+ PERIOD,
+ BIT_OR,
+ BIT_AND,
+ BIT_XOR,
+ BIT_NOT,
+ NOT,
+ LT,
+ GT,
+ MOD,
+ ASSIGN,
+ ADD,
+ SUB,
+ MUL,
+ DIV,
+ PRIVATE_NAME,
+ STRING,
+ TEMPLATE_SPAN,
+ IDENTIFIER,
+ WHITESPACE,
+ ILLEGAL,
+};
+
+constexpr Value GetOneCharToken(char c) {
+ return
+ c == '(' ? LPAREN :
+ c == ')' ? RPAREN :
+ c == '{' ? LBRACE :
+ c == '}' ? RBRACE :
+ c == '[' ? LBRACK :
+ c == ']' ? RBRACK :
+ c == '?' ? CONDITIONAL :
+ c == ':' ? COLON :
+ c == ';' ? SEMICOLON :
+ c == ',' ? COMMA :
+ c == '.' ? PERIOD :
+ c == '|' ? BIT_OR :
+ c == '&' ? BIT_AND :
+ c == '^' ? BIT_XOR :
+ c == '~' ? BIT_NOT :
+ c == '!' ? NOT :
+ c == '<' ? LT :
+ c == '>' ? GT :
+ c == '%' ? MOD :
+ c == '=' ? ASSIGN :
+ c == '+' ? ADD :
+ c == '-' ? SUB :
+ c == '*' ? MUL :
+ c == '/' ? DIV :
+ c == '#' ? PRIVATE_NAME :
+ c == '"' ? STRING :
+ c == '\'' ? STRING :
+ c == '`' ? TEMPLATE_SPAN :
+ c == '\\' ? IDENTIFIER :
+ c == ' ' ? WHITESPACE :
+ c == '\t' ? WHITESPACE :
+ c == '\v' ? WHITESPACE :
+ c == '\f' ? WHITESPACE :
+ c == '\r' ? WHITESPACE :
+ c == '\n' ? WHITESPACE :
+ ILLEGAL;
+}
+
+int main() {}
diff --git a/gcc/testsuite/g++.dg/cpp2a/consteval36.C
b/gcc/testsuite/g++.dg/cpp2a/consteval36.C
new file mode 100644
index 00000000000..9c470e4b7d7
--- /dev/null
+++ b/gcc/testsuite/g++.dg/cpp2a/consteval36.C
@@ -0,0 +1,22 @@
+// PR c++/111660
+// { dg-do compile { target c++20 } }
+
+consteval int id (int i) { return i; }
+
+void
+g (int i)
+{
+ 1 ? 1 : ((1 ? 1 : 1), id (i)); // { dg-error "'i' is not a constant
expression" }
+ 1 ? 1 : ((1 ? 1 : 1), id (i), 1); // { dg-error "'i' is not a constant
expression" }
+ 1 ? 1 : ((i ? 1 : 1), id (i), 1); // { dg-error "'i' is not a constant
expression" }
+ 1 ? 1 : ((1 ? i : 1), id (i), 1); // { dg-error "'i' is not a constant
expression" }
+ 1 ? 1 : ((1 ? 1 : i), id (i), 1); // { dg-error "'i' is not a constant
expression" }
+ 1 ? 1 : ((i ? -i : i), id (i), 1); // { dg-error "'i' is not a constant
expression" }
+ 1 ? 1 : ((1 ? 1 : id (i)), id (42), 1); // { dg-error "'i' is not a constant
expression" }
+ 1 ? 1 : ((1 ? 1 : id (42)), id (i)); // { dg-error "'i' is not a constant
expression" }
+ 1 ? 1 : ((1 ? 1 : id (42)), id (i), 1); // { dg-error "'i' is not a constant
expression" }
+ id (i) ? 1 : ((1 ? 1 : 1), id (i)); // { dg-error "'i' is not a constant
expression" }
+ 1 ? 1 : ((1 ? 1 : id (i)), id (i)); // { dg-error "'i' is not a constant
expression" }
+ 1 ? id (i) : ((1 ? 1 : id (i)), id (i)); // { dg-error "'i' is not a
constant expression" }
+ 1 ? 1 : ((id (i) ? 1 : 1), id (i)); // { dg-error "'i' is not a constant
expression" }
+}
base-commit: 328745607c5d403a1c7b6bc2ecaa1574ee42122f
--
2.41.0
