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From: Michael Meissner <meissner@gcc.gnu.org> To: gcc-cvs@gcc.gnu.org Subject: [gcc(refs/users/meissner/heads/work139)] PowerPC: Do not depend on an undefined shift Date: Thu, 12 Oct 2023 21:54:03 +0000 (GMT) [thread overview] Message-ID: <20231012215403.1B4BB385AE4E@sourceware.org> (raw) https://gcc.gnu.org/g:30673dbfa939bd034e12d7998a7c5a5d15ad3f27 commit 30673dbfa939bd034e12d7998a7c5a5d15ad3f27 Author: Michael Meissner <meissner@linux.ibm.com> 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 <guojiufu@linux.ibm.com> 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 <meissner@linux.ibm.com> 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 cc24dd5301e..6ffd982c7bc 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;
next reply other threads:[~2023-10-12 21:54 UTC|newest] Thread overview: 7+ messages / expand[flat|nested] mbox.gz Atom feed top 2023-10-12 21:54 Michael Meissner [this message] -- strict thread matches above, loose matches on Subject: below -- 2023-10-16 18:24 Michael Meissner 2023-10-16 18:23 Michael Meissner 2023-10-16 18:23 Michael Meissner 2023-10-12 21:54 Michael Meissner 2023-10-12 4:49 Michael Meissner 2023-10-12 4:07 Michael Meissner
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