From 4ce7c81212c7819dfe6dbbe2399220fb12da6d71 Mon Sep 17 00:00:00 2001 From: "Maciej W. Rozycki" Date: Sat, 29 Jun 2024 23:26:55 +0100 Subject: [PR115565] cse: Don't use a valid regno for non-register in comparison_qty Use INT_MIN rather than -1 in `comparison_qty' where a comparison is not with a register, because the value of -1 is actually a valid reference to register 0 in the case where it has not been assigned a quantity. Using -1 makes `REG_QTY (REGNO (folded_arg1)) == ent->comparison_qty' comparison in `fold_rtx' to incorrectly trigger in rare circumstances and return true for a memory reference, making CSE consider a comparison operation to evaluate to a constant expression and consequently make the resulting code incorrectly execute or fail to execute conditional blocks. This has caused a miscompilation of rwlock.c from LinuxThreads for the `alpha-linux-gnu' target, where `rwlock->__rw_writer != thread_self ()' expression (where `thread_self' returns the thread pointer via a PALcode call) has been decided to be always true (with `ent->comparison_qty' using -1 for a reference to to `rwlock->__rw_writer', while register 0 holding the thread pointer retrieved by `thread_self') and code for the false case has been optimized away where it mustn't have, causing program lockups. The issue has been observed as a regression from commit 08a692679fb8 ("Undefined cse.c behaviour causes 3.4 regression on HPUX"), , and up to commit 932ad4d9b550 ("Make CSE path following use the CFG"), , where CSE has been restructured sufficiently for the issue not to trigger with the original reproducer anymore. However the original bug remains and can trigger, because `comparison_qty' will still be assigned -1 for a memory reference and the `reg_qty' member of a `cse_reg_info_table' entry will still be assigned -1 for register 0 where the entry has not been assigned a quantity, e.g. at initialization. Use INT_MIN then as noted above, so that the value remains negative, for consistency with the REGNO_QTY_VALID_P macro (even though not used on `comparison_qty'), and then so that it should not ever match a valid negated register number, fixing the regression with commit 08a692679fb8. gcc/ PR rtl-optimization/115565 * cse.cc (record_jump_cond): Use INT_MIN rather than -1 for `comparison_qty' if !REG_P. (cherry picked from commit 69bc5fb97dc3fada81869e00fa65d39f7def6acf) --- gcc/cse.cc | 4 ++-- 1 file changed, 2 insertions(+), 2 deletions(-) diff --git a/gcc/cse.cc b/gcc/cse.cc index 8fbda4e..9a399e3 100644 --- a/gcc/cse.cc +++ b/gcc/cse.cc @@ -239,7 +239,7 @@ static int next_qty; the constant being compared against, or zero if the comparison is not against a constant. `comparison_qty' holds the quantity being compared against when the result is known. If the comparison - is not with a register, `comparison_qty' is -1. */ + is not with a register, `comparison_qty' is INT_MIN. */ struct qty_table_elem { @@ -4068,7 +4068,7 @@ record_jump_cond (enum rtx_code code, machine_mode mode, rtx op0, else { ent->comparison_const = op1; - ent->comparison_qty = -1; + ent->comparison_qty = INT_MIN; } return; -- cgit v1.1