From mboxrd@z Thu Jan 1 00:00:00 1970 Return-Path: Received: by sourceware.org (Postfix, from userid 1005) id DB2B43858D1E; Thu, 12 Oct 2023 04:07:51 +0000 (GMT) DKIM-Filter: OpenDKIM Filter v2.11.0 sourceware.org DB2B43858D1E DKIM-Signature: v=1; a=rsa-sha256; c=relaxed/relaxed; d=gcc.gnu.org; s=default; t=1697083671; bh=nNCx0bzUZqKDm0qLN5FDN7X2O0Dq4m6FrhWGJQURiNY=; h=From:To:Subject:Date:From; b=Ccvxq2U2aDOUjErLzChc/EXSrTo11sJUSaHK4NbgC4aofIVCW3v9hwj8j9aj1mUcP Xh1kPInhCUq2bUfftWVlWqwTuZCEQw1673u+RLiG8Tr1MIzlmpDWVPwWEhJCYvcf9p 1S9CJFMLBHFT9ExLIvbvdyqXjcKhHry88OgBhi0M= 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: f44977e5c6f198ec03127063ad570bf1712fdfdd X-Git-Newrev: 0016f87ad08d0218a51c091334c9372ed6dfa497 Message-Id: <20231012040751.DB2B43858D1E@sourceware.org> Date: Thu, 12 Oct 2023 04:07:51 +0000 (GMT) List-Id: https://gcc.gnu.org/g:0016f87ad08d0218a51c091334c9372ed6dfa497 commit 0016f87ad08d0218a51c091334c9372ed6dfa497 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 | 11 +++++++++++ 1 file changed, 11 insertions(+) diff --git a/gcc/config/rs6000/rs6000.cc b/gcc/config/rs6000/rs6000.cc index 2828f01413c..b03913fe71b 100644 --- a/gcc/config/rs6000/rs6000.cc +++ b/gcc/config/rs6000/rs6000.cc @@ -10370,6 +10370,17 @@ can_be_built_by_li_lis_and_rldicl (HOST_WIDE_INT c, int *shift, /* Leading zeros may be cleaned by rldicl with a mask. Change leading zeros to ones and then recheck it. */ int lz = clz_hwi (c); + + /* 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. */ + if (!lz) + return false; + HOST_WIDE_INT unmask_c = c | (HOST_WIDE_INT_M1U << (HOST_BITS_PER_WIDE_INT - lz)); int n;