Re: C++ ptrmemfun break if FUNCTION_BOUNDARY < 2 * BITS_PER_UNIT

Alexandre Oliva
2001-04-09T13:12:00Z

On Apr 9, 2001, Joern Rennecke <amylaar@redhat.com> wrote:

> Is it safe to use arithmetic SHIFTs instead of letting the compiler
>> choose them instead of DIVs and MULTs? For some reason, on
>> mn10300-elf, the div makes it to the generated code, which we
>> certainly don't want.

> A signed division is not a portable way to extract a bit.

I don't want to extract a bit. I want to extract the remaining bits.
The delta was shifted left (or multiplied by 2) to make room for the
vbit; now I want the delta back. It's a signed offset. The question
is : does GCC's notion of arithmetic shift, in the tree level,
guarantee sign-extension semantics, or do we really have to use
TRUNC_DIV, and I have to figure out why the heck it's not being
converted to an arithmetic shift on mn10300-elf, given that the asr
opcode does exactly the right thing.

> And for negative values, it just doesn't work.

Doh. Never mind. I *does* work. It's TRUNC_DIV that doesn't, for
what I need. I really need a right shift with sign extension:

3 >> 1 == 1
2 >> 1 == 1
1 >> 1 == 0
0 >> 1 == 0
0xffff >> 1 == 0xffff
0xfffe >> 1 == 0xffff
0xfffd >> 1 == 0xfffe
0xfffc >> 1 == 0xfffe

Does RSHIFT_EXPR guarantee the preservation of the most significant
(sign) bit? Does it guarantee the semantics above?

--
Alexandre Oliva Enjoy Guarana', see http://www.ic.unicamp.br/~oliva/
Red Hat GCC Developer aoliva@{cygnus.com, redhat.com}
CS PhD student at IC-Unicamp oliva@{lsd.ic.unicamp.br, gnu.org}
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