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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 04:49:52 +0000 (GMT) [thread overview] Message-ID: <20231012044952.8E0473858D1E@sourceware.org> (raw) https://gcc.gnu.org/g:49fb3f85c907777381cc5dde79dda2fc90c8dc9c commit 49fb3f85c907777381cc5dde79dda2fc90c8dc9c Author: Michael Meissner <meissner@linux.ibm.com> Date: Thu Oct 12 00:49:35 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): Protect code from shifts that are undefined. (can_be_built_by_li_lis_and_rldicr): Likewise. (can_be_built_by_li_and_rldic): Likewise. Diff: --- gcc/config/rs6000/rs6000.cc | 29 ++++++++++++++++++++++++++--- 1 file changed, 26 insertions(+), 3 deletions(-) diff --git a/gcc/config/rs6000/rs6000.cc b/gcc/config/rs6000/rs6000.cc index 2828f01413c..cc24dd5301e 100644 --- a/gcc/config/rs6000/rs6000.cc +++ b/gcc/config/rs6000/rs6000.cc @@ -10370,6 +10370,11 @@ 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); + + /* If lz == 0, the left shift is undefined. */ + if (!lz) + return false; + HOST_WIDE_INT unmask_c = c | (HOST_WIDE_INT_M1U << (HOST_BITS_PER_WIDE_INT - lz)); int n; @@ -10398,6 +10403,11 @@ can_be_built_by_li_lis_and_rldicr (HOST_WIDE_INT c, int *shift, /* Tailing zeros may be cleaned by rldicr with a mask. Change tailing zeros to ones and then recheck it. */ int tz = ctz_hwi (c); + + /* If tz == HOST_BITS_PER_WIDE_INT, the left shift is undefined. */ + if (tz >= HOST_BITS_PER_WIDE_INT) + return false; + HOST_WIDE_INT unmask_c = c | ((HOST_WIDE_INT_1U << tz) - 1); int n; if (can_be_rotated_to_lowbits (~unmask_c, 15, &n) @@ -10428,8 +10438,15 @@ can_be_built_by_li_and_rldic (HOST_WIDE_INT c, int *shift, HOST_WIDE_INT *mask) right bits are shifted as 0's, and left 1's(and x's) are cleaned. */ int tz = ctz_hwi (c); int lz = clz_hwi (c); + + /* If lz == HOST_BITS_PER_WIDE_INT, the left shift is undefined. */ + if (lz >= HOST_BITS_PER_WIDE_INT) + return false; + int middle_ones = clz_hwi (~(c << lz)); - if (tz + lz + middle_ones >= ones) + if (tz + lz + middle_ones >= ones + && (tz - lz) < HOST_BITS_PER_WIDE_INT + && tz < HOST_BITS_PER_WIDE_INT) { *mask = ((1LL << (HOST_BITS_PER_WIDE_INT - tz - lz)) - 1LL) << tz; *shift = tz; @@ -10440,7 +10457,8 @@ can_be_built_by_li_and_rldic (HOST_WIDE_INT c, int *shift, HOST_WIDE_INT *mask) int leading_ones = clz_hwi (~c); int tailing_ones = ctz_hwi (~c); int middle_zeros = ctz_hwi (c >> tailing_ones); - if (leading_ones + tailing_ones + middle_zeros >= ones) + if (leading_ones + tailing_ones + middle_zeros >= ones + && middle_zeros < HOST_BITS_PER_WIDE_INT) { *mask = ~(((1ULL << middle_zeros) - 1ULL) << tailing_ones); *shift = tailing_ones + middle_zeros; @@ -10450,10 +10468,15 @@ can_be_built_by_li_and_rldic (HOST_WIDE_INT c, int *shift, HOST_WIDE_INT *mask) /* xx1..1xx: --> xx0..01..1xx: some 1's(following x's) are cleaned. */ /* Get the position for the first bit of successive 1. The 24th bit would be in successive 0 or 1. */ - HOST_WIDE_INT low_mask = (1LL << 24) - 1LL; + HOST_WIDE_INT low_mask = (HOST_WIDE_INT_1U << 24) - HOST_WIDE_INT_1U; int pos_first_1 = ((c & (low_mask + 1)) == 0) ? clz_hwi (c & low_mask) : HOST_BITS_PER_WIDE_INT - ctz_hwi (~(c | low_mask)); + + /* Make sure the left and right shifts are defined. */ + if (!IN_RANGE (pos_first_1, 1, HOST_BITS_PER_WIDE_INT-1)) + return false; + middle_ones = clz_hwi (~c << pos_first_1); middle_zeros = ctz_hwi (c >> (HOST_BITS_PER_WIDE_INT - pos_first_1)); if (pos_first_1 < HOST_BITS_PER_WIDE_INT
next reply other threads:[~2023-10-12 4:49 UTC|newest] Thread overview: 7+ messages / expand[flat|nested] mbox.gz Atom feed top 2023-10-12 4:49 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 21:54 Michael Meissner 2023-10-12 4:07 Michael Meissner
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