Messages in this thread Patch in this message | | | Date | Wed, 3 Apr 2019 05:45:26 GMT | From | George Spelvin <> | Subject | [PATCH v2] ubsan: Avoid unnecessary 128-bit shifts |
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If CONFIG_ARCH_SUPPORTS_INT128, s_max is 128 bits, and variable sign-extending shifts of such a double-word data type are a non-trivial amount of code and complexity. Do a single-word shift *before* the cast to (s_max), greatly simplifying the object code.
(Yes, I know "signed long" is redundant. It's there for emphasis.)
On s390 (and perhaps some other arches), gcc implements variable 128-bit shifts using an __ashrti3 helper function which the kernel doesn't provide, causing a link error. In that case, this patch is a prerequisite for enabling INT128 support. Andrey Ryabinin has gven permission for any arch that needs it to cherry-pick it so they don't have to wait for ubsan to be merged into Linus' tree.
We *could*, alternatively, implement __ashrti3, but that becomes dead as soon as this patch is merged, so it seems like a waste of time and its absence discourages people from adding inefficient code. Note that the shifts in <math64.h> (unsigned, and by a compile-time constant amount) are simpler and generated inline.
Signed-off-by: George Spelvin <lkml@sdf.org> Acked-By: Andrey Ryabinin <aryabinin@virtuozzo.com> Cc: linux-s390@vger.kernel.org Cc: Heiko Carstens <heiko.carstens@de.ibm.com> --- lib/ubsan.c | 4 ++-- 1 file changed, 2 insertions(+), 2 deletions(-)
v1->v2: Eliminated redundant cast to (s_max). Rewrote commit message without "is this the right thing to do?" verbiage. Incorporated ack from Andrey Ryabinin.
diff --git a/lib/ubsan.c b/lib/ubsan.c index e4162f59a81c..a7eb55fbeede 100644 --- a/lib/ubsan.c +++ b/lib/ubsan.c @@ -89,8 +89,8 @@ static bool is_inline_int(struct type_descriptor *type) static s_max get_signed_val(struct type_descriptor *type, unsigned long val) { if (is_inline_int(type)) { - unsigned extra_bits = sizeof(s_max)*8 - type_bit_width(type); - return ((s_max)val) << extra_bits >> extra_bits; + unsigned extra_bits = sizeof(val)*8 - type_bit_width(type); + return (signed long)val << extra_bits >> extra_bits; } if (type_bit_width(type) == 64) -- 2.20.1
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