Specify signed/unsigned/dontcare in calls to extract_bit_field_1.

Specify signed/unsigned/dontcare in calls to extract_bit_field_1.

This patch is inspired by Jakub's work on PR rtl-optimization/110717.
The bitfield example described in comment #2, looks like:

struct S { __int128 a : 69; };
unsigned type bar (struct S *p) {
  return p->a;
}

which on x86_64 with -O2 currently generates:

bar:    movzbl  8(%rdi), %ecx
        movq    (%rdi), %rax
        andl    $31, %ecx
        movq    %rcx, %rdx
        salq    $59, %rdx
        sarq    $59, %rdx
        ret

The ANDL $31 is interesting... we first extract an unsigned 69-bit bitfield
by masking/clearing the top bits of the most significant word, and then
it gets sign-extended, by left shifting and arithmetic right shifting.
Obviously, this bit-wise AND is redundant, for signed bit-fields, we don't
require these bits to be cleared, if we're about to set them appropriately.

This patch eliminates this redundancy in the middle-end, during RTL
expansion, but extending the extract_bit_field APIs so that the integer
UNSIGNEDP argument takes a special value; 0 indicates the field should
be sign extended, 1 (any non-zero value) indicates the field should be
zero extended, but -1 indicates a third option, that we don't care how
or whether the field is extended.  By passing and checking this sentinel
value at the appropriate places we avoid the useless bit masking (on
all targets).

For the test case above, with this patch we now generate:

bar:    movzbl  8(%rdi), %ecx
        movq    (%rdi), %rax
        movq    %rcx, %rdx
        salq    $59, %rdx
        sarq    $59, %rdx
        ret

2023-08-04  Roger Sayle  <roger@nextmovesoftware.com>

gcc/ChangeLog
	* expmed.cc (extract_bit_field_1): Document that an UNSIGNEDP
	value of -1 is equivalent to don't care.
	(extract_integral_bit_field): Indicate that we don't require
	the most significant word to be zero extended, if we're about
	to sign extend it.
	(extract_fixed_bit_field_1): Document that an UNSIGNEDP value
	of -1 is equivalent to don't care.  Don't clear the most
	significant bits with AND mask when UNSIGNEDP is -1.

gcc/testsuite/ChangeLog
	* gcc.target/i386/pr110717-2.c: New test case.