Hi,
Thanks for your quick fix!
Michael Meissner <meiss...@linux.ibm.com> writes:
> 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 <guoji...@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.
>
> Can I check this into the master branch to fix the problem?
>
> 2023-10-12 Michael Meissner <meiss...@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): Protect code from shifts that
> undefined. Also replace uses of 1ULL with HOST_WIDE_INT_1U.
>
> ---
> 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;
> +
Thanks! This should be checked.
If "lz" is zero, it means for input "C", there is no leading
zeros which are cleanded by "rldicl". And then no future analyzing is
needed.
> 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;
> +
This is correct in theory and could make sure "tz" is ok.
Just one minor thing:
"ctz_hwi" would not return value greater than HOST_BITS_PER_WIDE_INT
other than 0, right?
> 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;
> +
This maybe similar.
> 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)
Thanks.
> {
> *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;
Yes.
> 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;
> +
Yes, this change would be safer.
Thanks again for the enhancement!
BR,
Jeff (Jiufu Guo)
> 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
> --
> 2.41.0
