match.pd: Fix up A % (cast) (pow2cst << B) simplification [PR99079]

match.pd: Fix up A % (cast) (pow2cst << B) simplification [PR99079]

The (mod @0 (convert?@3 (power_of_two_cand@1 @2))) simplification
uses tree_nop_conversion_p (type, TREE_TYPE (@3)) condition, but I believe
it doesn't check what it was meant to check.  On convert?@3
TREE_TYPE (@3) is not the type of what it has been converted from, but
what it has been converted to, which needs to be (because it is operand
of normal binary operation) equal or compatible to type of the modulo
result and first operand - type.
I could fix that by using && tree_nop_conversion_p (type, TREE_TYPE (@1))
and be done with it, but actually most of the non-nop conversions are IMHO
ok and so we would regress those optimizations.
In particular, if we have say narrowing conversions (foo5 and foo6 in
the new testcase), I think we are fine, either the shift of the power of two
constant after narrowing conversion is still that power of two (or negation
of that) and then it will still work, or the result of narrowing conversion
is 0 and then we would have UB which we can ignore.
Similarly, widening conversions where the shift result is unsigned are fine,
or even widening conversions where the shift result is signed, but we sign
extend to a signed wider divisor, the problematic case of INT_MIN will
become x % (long long) INT_MIN and we can still optimize that to
x & (long long) INT_MAX.
What doesn't work is the case in the pr99079.c testcase, widening conversion
of a signed shift result to wider unsigned divisor, where if the shift
is negative, we end up with x % (unsigned long long) INT_MIN which is
x % 0xffffffff80000000ULL where the divisor is not a power of two and
we can't optimize that to x & 0x7fffffffULL.

So, the patch rejects only the single problematic case.

Furthermore, when the shift result is signed, we were introducing UB into
a program which previously didn't have one (well, left shift into the sign
bit is UB in some language/version pairs, but it is definitely valid in
C++20 - wonder if I shouldn't move the gcc.c-torture/execute/pr99079.c
testcase to g++.dg/torture/pr99079.C and use -std=c++20), by adding that
subtraction of 1, x % (1 << 31) in C++20 is well defined, but
x & ((1 << 31) - 1) triggers UB on the subtraction.
So, the patch performs the subtraction in the unsigned type if it isn't
wrapping.

2021-02-15  Jakub Jelinek  <jakub@redhat.com>

	PR tree-optimization/99079
	* match.pd (A % (pow2pcst << N) -> A & ((pow2pcst << N) - 1)): Remove
	useless tree_nop_conversion_p (type, TREE_TYPE (@3)) check.  Instead
	require both type and TREE_TYPE (@1) to be integral types and either
	type having smaller or equal precision, or TREE_TYPE (@1) being
	unsigned type, or type being signed type.  If TREE_TYPE (@1)
	doesn't have wrapping overflow, perform the subtraction of one in
	unsigned type.

	* gcc.dg/fold-modpow2-2.c: New test.
	* gcc.c-torture/execute/pr99079.c: New test.