Re: [PATCH] PR tree-opt/113673: Avoid load merging from potentially trapping additions.

[Richard Biener <richard.guenther@gmail.com>]

On Sun, Apr 28, 2024 at 11:11 AM Roger Sayle <roger@nextmovesoftware.com> wrote:
>
>
> This patch fixes PR tree-optimization/113673, a P2 ice-on-valid regression
> caused by load merging of (ptr[0]<<8)+ptr[1] when -ftrapv has been
> specified.  When the operator is | or ^ this is safe, but for addition
> of signed integer types, a trap may be generated/required, so merging this
> idiom into a single non-trapping instruction is inappropriate, confusing
> the compiler by transforming a basic block with an exception edge into one
> without.  One fix is to be more selective for PLUS_EXPR than for
> BIT_IOR_EXPR or BIT_XOR_EXPR in gimple-ssa-store-merging.cc's
> find_bswap_or_nop_1 function.
>
> An alternate solution might be to notice that in this idiom the addition
> can't overflow, but that this detail wasn't apparent when exception edges
> were added to the CFG.  In which case, it's safe to remove (or mark for
> removal) the problematic exceptional edge.  Unfortunately updating the
> CFG is a part of the compiler that I'm less familiar with.
>
> This patch has been tested on x86_64-pc-linux-gnu with make bootstrap
> and make -k check, both with and without --target_board=unix{-m32}
> with no new failures.  Ok for mainline?

Instead of

+       case PLUS_EXPR:
+         /* Don't perform load merging if this addition can trap.  */
+         if (cfun->can_throw_non_call_exceptions
+             && INTEGRAL_TYPE_P (TREE_TYPE (rhs1))
+             && TYPE_OVERFLOW_TRAPS (TREE_TYPE (rhs1)))
+           return NULL;

please check stmt_can_throw_internal (cfun, stmt) - the find_bswap_or_no_load
call in the function suffers from the same issue, so this should probably
be checked before that call even.

Thanks,
Richard.

>
> 2024-04-28  Roger Sayle  <roger@nextmovesoftware.com>
>
> gcc/ChangeLog
>         PR tree-optimization/113673
>         * gimple-ssa-store-merging.cc (find_bswap_or_nop_1) <case
> PLUS_EXPR>:
>         Don't perform load merging if a signed addition may trap.
>
> gcc/testsuite/ChangeLog
>         PR tree-optimization/113673
>         * g++.dg/pr113673.C: New test case.
>
>
> Thanks in advance,
> Roger
> --
>