From mboxrd@z Thu Jan 1 00:00:00 1970 Return-Path: Received: by sourceware.org (Postfix, from userid 1005) id 2D5EA385B537; Mon, 16 Oct 2023 18:23:41 +0000 (GMT) DKIM-Filter: OpenDKIM Filter v2.11.0 sourceware.org 2D5EA385B537 DKIM-Signature: v=1; a=rsa-sha256; c=relaxed/relaxed; d=gcc.gnu.org; s=default; t=1697480621; bh=boYxoay/Wgu2YsD1vRYg6aOknJYhudF3EVAKCo6fTHI=; h=From:To:Subject:Date:From; b=Qh26iwi2PvfmsS+TASF4kmfxFrI9XP0cP8sxxe27GvRzdmkvSwax0zGRLxh6qQORs x2fVZrmDuhyTft2qFAFBdyX+G8jOHXDBEIFULJPizx/JoV/z0QZKHjbfXNnfdyvVP/ mcO2n8vPO2Z8EzfhfW2/JQlAG8fOMzHV34IpfmQU= Content-Type: text/plain; charset="us-ascii" MIME-Version: 1.0 Content-Transfer-Encoding: 7bit From: Michael Meissner To: gcc-cvs@gcc.gnu.org Subject: [gcc(refs/users/meissner/heads/work139)] PowerPC: Do not depend on an undefined shift X-Act-Checkin: gcc X-Git-Author: Michael Meissner X-Git-Refname: refs/users/meissner/heads/work139 X-Git-Oldrev: 05f2069b31a747513f47521236ba53f09b3720f5 X-Git-Newrev: 232f13ca9075f9ad33e6f62def03e3d6032462ce Message-Id: <20231016182341.2D5EA385B537@sourceware.org> Date: Mon, 16 Oct 2023 18:23:41 +0000 (GMT) List-Id: https://gcc.gnu.org/g:232f13ca9075f9ad33e6f62def03e3d6032462ce commit 232f13ca9075f9ad33e6f62def03e3d6032462ce Author: Michael Meissner Date: Thu Oct 12 00:07:36 2023 -0400 PowerPC: Do not depend on an undefined shift I was building a cross compiler to PowerPC on my x86_86 workstation with the latest version of GCC on October 11th. I could not build the compiler on the x86_64 system as it died in building libgcc. I looked into it, and I discovered the compiler was recursing until it ran out of stack space. If I build a native compiler with the same sources on a PowerPC system, it builds fine. I traced this down to a change made around October 10th: commit 8f1a70a4fbcc6441c70da60d4ef6db1e5635e18a (HEAD) Author: Jiufu Guo Date: Tue Jan 10 20:52:33 2023 +0800 rs6000: build constant via li/lis;rldicl/rldicr If a constant is possible left/right cleaned on a rotated value from a negative value of "li/lis". Then, using "li/lis ; rldicl/rldicr" to build the constant. The code was doing a -1 << 64 which is undefined behavior because different machines produce different results. On the x86_64 system, (-1 << 64) produces -1 while on a PowerPC 64-bit system, (-1 << 64) produces 0. The x86_64 then recurses until the stack runs out of space. If I apply this patch, the compiler builds fine on both x86_64 as a PowerPC crosss compiler and on a native PowerPC system. 2023-10-12 Michael Meissner gcc/ PR target/111778 * config/rs6000/rs6000.cc (can_be_built_by_li_lis_and_rldicl): Do not use a shift left by the number of bits in a host wide int. Diff: --- gcc/config/rs6000/rs6000.cc | 10 ++++++++++ 1 file changed, 10 insertions(+) diff --git a/gcc/config/rs6000/rs6000.cc b/gcc/config/rs6000/rs6000.cc index cc24dd5301e3..6ffd982c7bcf 100644 --- a/gcc/config/rs6000/rs6000.cc +++ b/gcc/config/rs6000/rs6000.cc @@ -10371,7 +10371,17 @@ can_be_built_by_li_lis_and_rldicl (HOST_WIDE_INT c, int *shift, to ones and then recheck it. */ int lz = clz_hwi (c); +<<<<<<< HEAD /* If lz == 0, the left shift is undefined. */ +======= + /* Different machines interpret num << shift differently if shift is at least + the number of bits in num's representation. It is explicitly undefined + behavior in the C/C++ langauges. + + In particular (-1 << 64) on an x86_64 produces -1 and (-1 << 64) on a + 64-bit PowerPC produces 0. This difference causes a cross compiler on + x86_64 to recurse until it runs out of stack. */ +>>>>>>> 0016f87ad08 (PowerPC: Do not depend on an undefined shift) if (!lz) return false;