On Tue, 2 Dec 2014, Richard Biener wrote:
> > I'm not sure if these forms of division actually occur in places where
> > this could cause a problem, but it does look like Ada may enable you to
> > generate ROUND_DIV_EXPR.
>
> Hmm. I thought I was following what extract_muldiv_1 does (but of course
> that function is quite hard to follow).
I don't claim existing optimizations are correct for all these forms of
division.
In fact even wide-int doesn't handle ROUND_DIV_EXPR correctly (round to
nearest, ties away from 0) - at a glance, without testing, ROUND_MOD_EXPR
has the same bug (wi::ges_p (wi::abs (remainder), wi::lrshift (wi::abs
(y), 1)) is not the right condition for rounding away). The following Ada
testcase test_round_div.adb will generate a ROUND_DIV_EXPR which is
executed correctly at -O0, but fails at -O3. (The separate division
function seems necessary to stop the front end from doing its own constant
folding, though there may for all I know be more idiomatic Ada ways of
doing that.)
procedure Test_Round_Div is
type Fixed is delta 1.0 range -2147483648.0 .. 2147483647.0;
A : Fixed := 1.0;
B : Fixed := 3.0;
C : Integer;
function Divide (X, Y : Fixed) return Integer is
begin
return Integer (X / Y);
end;
begin
C := Divide (A, B);
if C /= 0 then
raise Program_Error;
end if;
end Test_Round_Div;
--
Joseph S. Myers
jos...@codesourcery.com
