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[PATCH] PR49720, infinite recursion in RTX simplification

this patch is for a specific case in RTX simplification where infinite
recursion ensues, causing the out-of-stack segfault in PR49720.

Tracking back the origin of this bug, the exact revision causing this
was rev.92429, back in 2004.

The clause in simplify-rtx.c:simplify_relational_operation_1(),
transforming X + cst1 == cst2 into X == cst2 - cst1 can cause an
oscillating recursive cycle like this:

  A + B == C    <===>   C - B == A

When all A, B, and C are CONSTANT_P, but not simplifiable (in the PR
testcase, they are all SYMBOL_REFs).  The switching of XEXP(op0,1) to
the other side of the equation, plus commutative canonicalizing then
completes the cycle.

This can be solved by using simplify_binary_relation() and returning
NULL_RTX when simplification of the constant PLUS/MINUS fails, but I'm
not sure that further working on X == CST2-CST1 will never yield more
optimization, so the patch here just tries to plug the recursion hole
for the exact condition.

Bootstrapped and tested on i686 and x86_64 without regressions. Is this
okay for trunk?


2011-10-27  Chung-Lin Tang  <>

	PR rtl-optimization/49720
	* simplify-rtx.c (simplify_relational_operation_1): Detect
	infinite recursion condition in "(eq/ne (plus x cst1) cst2)
	simplifies to (eq/ne x (cst2 - cst1))" case.

	* g++.dg/torture/pr49720.C: New test.

Attachment: pr49720.diff
Description: Text document

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