[{"id":3679116,"web_url":"http://patchwork.ozlabs.org/comment/3679116/","msgid":"","list_archive_url":null,"date":"2026-04-19T18:16:01","subject":"Re: [PATCH] cse: Validate subreg replacements [PR124643]","submitter":{"id":91428,"url":"http://patchwork.ozlabs.org/api/people/91428/","name":"Andrew Pinski","email":"andrew.pinski@oss.qualcomm.com"},"content":"On Sat, Apr 18, 2026 at 8:29 AM Richard Sandiford\n wrote:\n>\n> The PR is about an ICE on sh caused by an \"invalid\" subreg.\n> cse replaced a pseudo register with the hard T register within:\n>\n> (zero_extend:SI (subreg:QI (reg:SI pseudo) 3))\n>\n> Since this is a register-for-register replacement, cse just relied on\n> recog to reject anything that wasn't valid.\n>\n> However, if validate_subreg had been asked, it would have said that:\n>\n> (subreg:QI (reg:SI T) 3)\n>\n> is not valid. This means that even simplify_gen_subreg would have\n> refused to generate it.\n>\n> In that sense, cse should not even be trying to match this replacement.\n> It's not recog's job to reject all invalid rtl. recog is just supposed\n> to say whether the machine supports a given piece of valid rtl.\n>\n> In this particular case, the sh port does specifically match:\n>\n> (zero_extend:SI (subreg:QI (reg:SI T) 3))\n>\n> even though, by forbidding T from having QImode, the port also\n> effectively forbids the subreg. See the discussion in the PR trail\n> about that. But I think the point still stands that cse should verify\n> the subregs that it creates. It should also try to simplify them down\n> to hard registers where possible.\n>\n> I suppose a more complete fix would be to rewrite canon_reg to use a\n> helper that recursively replaces and simplifies, but that seems somewhat\n> dangerous at this stage. The scope for non-subreg simplification should\n> also be pretty limited in practice.\n>\n> Tested on x86_64-linux-gnu, powerpc64le-linux-gnu and aarch64-linux-gnu.\n> OK to install?\n\nOk.\n\n>\n> Richard\n>\n>\n> gcc/\n> PR rtl-optimization/124643\n> * cse.cc (canon_reg): Handle and canonicalize subregs.\n>\n> gcc/testsuite/\n> PR rtl-optimization/124643\n> * gcc.c-torture/compile/pr124643.c: New test.\n> ---\n> gcc/cse.cc | 18 ++++++++++++++++++\n> gcc/testsuite/gcc.c-torture/compile/pr124643.c | 8 ++++++++\n> 2 files changed, 26 insertions(+)\n> create mode 100644 gcc/testsuite/gcc.c-torture/compile/pr124643.c\n>\n> diff --git a/gcc/cse.cc b/gcc/cse.cc\n> index e5fa88f9de5..25b2bf500a9 100644\n> --- a/gcc/cse.cc\n> +++ b/gcc/cse.cc\n> @@ -2847,6 +2847,24 @@ canon_reg (rtx x, rtx_insn *insn)\n> case ADDR_DIFF_VEC:\n> return x;\n>\n> + case SUBREG:\n> + {\n> + rtx inner = canon_reg (SUBREG_REG (x), insn);\n> + if (inner != SUBREG_REG (x))\n> + {\n> + rtx newx = simplify_subreg (GET_MODE (x), inner,\n> + GET_MODE (SUBREG_REG (x)),\n> + SUBREG_BYTE (x));\n> + if (newx)\n> + return newx;\n> +\n> + if (validate_subreg (GET_MODE (x), GET_MODE (inner),\n> + inner, SUBREG_BYTE (x)))\n> + validate_change (insn, &SUBREG_REG (x), inner, 1);\n> + }\n> + return x;\n> + }\n> +\n> case REG:\n> {\n> int first;\n> diff --git a/gcc/testsuite/gcc.c-torture/compile/pr124643.c b/gcc/testsuite/gcc.c-torture/compile/pr124643.c\n> new file mode 100644\n> index 00000000000..f3f12bef11e\n> --- /dev/null\n> +++ b/gcc/testsuite/gcc.c-torture/compile/pr124643.c\n> @@ -0,0 +1,8 @@\n> +int a;\n> +char *b;\n> +void c(int a) {\n> + int d = (a & 2) != 0 && a & 1;\n> + for (;;)\n> + if (a)\n> + b[1 ^ d] = b[d];\n> +}\n> --\n> 2.53.0\n>","headers":{"Return-Path":"","X-Original-To":["incoming@patchwork.ozlabs.org","gcc-patches@gcc.gnu.org"],"Delivered-To":["patchwork-incoming@legolas.ozlabs.org","gcc-patches@gcc.gnu.org"],"Authentication-Results":["legolas.ozlabs.org;\n\tdkim=pass (2048-bit key;\n unprotected) header.d=qualcomm.com header.i=@qualcomm.com header.a=rsa-sha256\n header.s=qcppdkim1 header.b=nZhc4PGy;\n\tdkim=pass (2048-bit key;\n unprotected) header.d=oss.qualcomm.com header.i=@oss.qualcomm.com\n header.a=rsa-sha256 header.s=google header.b=hGmZmRHs;\n\tdkim-atps=neutral","legolas.ozlabs.org;\n spf=pass (sender SPF authorized) smtp.mailfrom=gcc.gnu.org\n (client-ip=38.145.34.32; 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