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tree: Improve skip_simple_arithmetic [PR119183]

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The following testcase takes very long time to compile, because
skip_simple_arithmetic decides to first call tree_invariant_p on
the second argument (and indirectly recurse there).  I think before
canonicalization of operands for commutative binary expressions
(and for non-commutative ones always) it is pretty common that the
first operand is a constant, something which tree_invariant_p handles
immediately, so the following patch special cases that; I've added
there a tree_invariant_p call too after the checks, while it is not
really needed currently, tree_invariant_p has the same checks, I wanted
to be prepared in case tree_invariant_p changes.  But if you think
I should avoid it, I can drop it too.

This is just a partial fix, I think one can certainly construct a testcase
which will still have horrible compile time complexity (but I've tried and
haven't managed to do so), so perhaps we should just limit the recursion
depth through skip_simple_arithmetic/tree_invariant_p with some defaulted
argument.

2025-03-11  Jakub Jelinek  <jakub@redhat.com>

	PR c/119183
	* tree.cc (skip_simple_arithmetic): If first operand of binary
	expr is TREE_CONSTANT or TREE_READONLY with no side-effects, call
	tree_invariant_p on that operand first instead of on the second.

	* gcc.dg/pr119183.c: New test.
This commit is contained in:
Jakub Jelinek 2025-03-11 11:01:55 +01:00 • committed by Jakub Jelinek
parent e1da6283a1
commit 20e5aa9cc1
2 changed files with 25 additions and 1 deletions
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12
gcc/testsuite/gcc.dg/pr119183.c Normal file
View file

@ -0,0 +1,12 @@
/* PR c/119183 */
/* { dg-do compile } */
int foo (void);
#define A(x) (1.0f * (1.0f * (1.0f * (1.0f * (1.0f * (1.0f * (1.0f * (1.0f * (x)))))))))
float
bar (float r)
{
r += A (A (A (A (A (A (A (A (foo ()))))))));
return r;
}

14
gcc/tree.cc
View file

@ -4105,7 +4105,19 @@ skip_simple_arithmetic (tree expr)
expr = TREE_OPERAND (expr, 0);
else if (BINARY_CLASS_P (expr))
{
if (tree_invariant_p (TREE_OPERAND (expr, 1)))
/* Before commutative binary operands are canonicalized,
it is quite common to have constants in the first operand.
Check for that common case first so that we don't walk
large expressions with tree_invariant_p unnecessarily.
This can still have terrible compile time complexity,
we should limit the depth of the tree_invariant_p and
skip_simple_arithmetic recursion. */
if ((TREE_CONSTANT (TREE_OPERAND (expr, 0))
|| (TREE_READONLY (TREE_OPERAND (expr, 0))
&& !TREE_SIDE_EFFECTS (TREE_OPERAND (expr, 0))))
&& tree_invariant_p (TREE_OPERAND (expr, 0)))
expr = TREE_OPERAND (expr, 1);
else if (tree_invariant_p (TREE_OPERAND (expr, 1)))
expr = TREE_OPERAND (expr, 0);
else if (tree_invariant_p (TREE_OPERAND (expr, 0)))
expr = TREE_OPERAND (expr, 1);