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The comment above simplify_rotate roughly describes what patterns
are matched into what:
We are looking for X with unsigned type T with bitsize B, OP being
+, | or ^, some type T2 wider than T. For:
(X << CNT1) OP (X >> CNT2) iff CNT1 + CNT2 == B
((T) ((T2) X << CNT1)) OP ((T) ((T2) X >> CNT2)) iff CNT1 + CNT2 == B
transform these into:
X r<< CNT1
Or for:
(X << Y) OP (X >> (B - Y))
(X << (int) Y) OP (X >> (int) (B - Y))
((T) ((T2) X << Y)) OP ((T) ((T2) X >> (B - Y)))
((T) ((T2) X << (int) Y)) OP ((T) ((T2) X >> (int) (B - Y)))
(X << Y) | (X >> ((-Y) & (B - 1)))
(X << (int) Y) | (X >> (int) ((-Y) & (B - 1)))
((T) ((T2) X << Y)) | ((T) ((T2) X >> ((-Y) & (B - 1))))
((T) ((T2) X << (int) Y)) | ((T) ((T2) X >> (int) ((-Y) & (B - 1))))
transform these into (last 2 only if ranger can prove Y < B):
X r<< Y
Or for:
(X << (Y & (B - 1))) | (X >> ((-Y) & (B - 1)))
(X << (int) (Y & (B - 1))) | (X >> (int) ((-Y) & (B - 1)))
((T) ((T2) X << (Y & (B - 1)))) | ((T) ((T2) X >> ((-Y) & (B - 1))))
((T) ((T2) X << (int) (Y & (B - 1)))) \
| ((T) ((T2) X >> (int) ((-Y) & (B - 1))))
transform these into:
X r<< (Y & (B - 1))
The following testcase shows that 2 of these are problematic.
If T2 is wider than T, then the 2 which yse (-Y) & (B - 1) on one
of the shift counts but Y on the can do something different from
rotate. E.g.:
__attribute__((noipa)) unsigned char
f7 (unsigned char x, unsigned int y)
{
unsigned int t = x;
return (t << y) | (t >> ((-y) & 7));
}
if y is [0, 7], then it is a normal rotate, and if y is in [32, ~0U]
then it is UB, but for y in [9, 31] the left shift in this case
will never leave any bits in the result, while in a rotate they are
left there. Say for y 5 and x 0xaa the expression gives
0x55 which is the same thing as rotate, while for y 19 and x 0xaa
0x5, which is different.
Now, I believe the
((T) ((T2) X << Y)) OP ((T) ((T2) X >> (B - Y)))
((T) ((T2) X << (int) Y)) OP ((T) ((T2) X >> (int) (B - Y)))
forms are ok, because B - Y still needs to be a valid shift count,
and if Y > B then B - Y should be either negative or very large
positive (for unsigned types).
And similarly the last 2 cases above which use & (B - 1) on both
shift operands are definitely ok.
The following patch disables the
((T) ((T2) X << Y)) | ((T) ((T2) X >> ((-Y) & (B - 1))))
((T) ((T2) X << (int) Y)) | ((T) ((T2) X >> (int) ((-Y) & (B - 1))))
unless ranger says Y is not in [B, B2 - 1] range.
And, looking at it again this morning, actually the Y equal to B
case is still fine, if Y is equal to 0, then it is
(T) (((T2) X << 0) | ((T2) X >> 0))
and so X, for Y == B it is
(T) (((T2) X << B) | ((T2) X >> 0))
which is the same as
(T) (0 | ((T2) X >> 0))
which is also X. So instead of the [B, B2 - 1] range we could use
[B + 1, B2 - 1]. And, if we wanted to go further, even multiplies
of B are ok if they are smaller than B2, so we could construct a detailed
int_range_max if we wanted.
2023-01-17 Jakub Jelinek <jakub@redhat.com>
PR tree-optimization/106523
* tree-ssa-forwprop.cc (simplify_rotate): For the
patterns with (-Y) & (B - 1) in one operand's shift
count and Y in another, if T2 has wider precision than T,
punt if Y could have a value in [B, B2 - 1] range.
* c-c++-common/rotate-2.c (f5, f6, f7, f8, f13, f14, f15, f16,
f37, f38, f39, f40, f45, f46, f47, f48): Add assertions using
__builtin_unreachable about shift count.
* c-c++-common/rotate-2b.c: New test.
* c-c++-common/rotate-4.c (f5, f6, f7, f8, f13, f14, f15, f16,
f37, f38, f39, f40, f45, f46, f47, f48): Add assertions using
__builtin_unreachable about shift count.
* c-c++-common/rotate-4b.c: New test.
* gcc.c-torture/execute/pr106523.c: New test.
(cherry picked from commit 001121e892)
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