Since this patch is sitting in the queue for quite some time and (more
importantly?) solves a bootstrap problem let me reiterate:
While writing the initial commit 7cdd0860949c6c3232e6cff1d7ca37bb5234074c
and the subsequent (potential) fix 41ef5a34161356817807be3a2e51fbdbe575ae85
I was not aware of the fact that the normal form of a CONST_INT,
representing an unsigned integer with fewer bits than in HOST_WIDE_INT,
is a sign-extended version of the unsigned integer. This invariant is
checked in rtl.h where we have at line 2297:
case CONST_INT:
if (precision < HOST_BITS_PER_WIDE_INT)
/* Nonzero BImodes are stored as STORE_FLAG_VALUE, which on many
targets is 1 rather than -1. */
gcc_checking_assert (INTVAL (x.first)
== sext_hwi (INTVAL (x.first), precision)
|| (x.second == BImode && INTVAL (x.first) == 1));
This was pretty surprising and frankly speaking unintuitive to me which
is why I was skimming further over existing code where I have found this
in combine.cc:
/* (unsigned) < 0x80000000 is equivalent to >= 0. */
else if (is_a <scalar_int_mode> (mode, &int_mode)
&& GET_MODE_PRECISION (int_mode) - 1 < HOST_BITS_PER_WIDE_INT
&& ((unsigned HOST_WIDE_INT) const_op
== HOST_WIDE_INT_1U << (GET_MODE_PRECISION (int_mode) - 1)))
{
The expression of the if-statement is a contradiction rendering the then-part
unreachable unless you mask out the upper bits of the sign-extended
unsigned integer const_op (as proposed in the inlined patch):
((unsigned HOST_WIDE_INT) const_op & GET_MODE_MASK (int_mode))
This is why I got a bit uncertain and hoped to get some feedback whether
my intuition is correct or not. Meanwhile I also found a comment in
the internals book at "14.7 Constant Expression Types" where we have:
"Constants generated for modes with fewer bits than in HOST_WIDE_INT
must be sign extended to full width (e.g., with gen_int_mode).
[...]
Note however that values are neither inherently signed nor
inherently unsigned; where necessary, signedness is determined by
the rtl operation instead."
At least this and the assert statement document that the normal form of
a CONST_INT is kind of special w.r.t. unsigned integers. Is there
anyone who can shed some light on _why_ such a normal form was chosen?
Independent of why such a normal form was chosen, this patch restores
the normal form and solves the bootstrap problem for Loongarch.
Cheers,
Stefan
