On 07/07/11 11:26, Andrew Stubbs wrote:
On 07/07/11 10:58, Richard Guenther wrote:
I think you should assume that series of widenings,
(int)(short)char_variable
are already combined. Thus I believe you only need to consider a single
conversion in valid_types_for_madd_p.
Hmm, I'm not so sure. I'll look into it a bit further.
OK, here's a test case that gives multiple conversions:
long long
foo (long long a, signed char b, signed char c)
{
int bc = b * c;
return a + (short)bc;
}
The dump right before the widen_mult pass gives:
foo (long long int a, signed char b, signed char c)
{
int bc;
long long int D.2018;
short int D.2017;
long long int D.2016;
int D.2015;
int D.2014;
<bb 2>:
D.2014_2 = (int) b_1(D);
D.2015_4 = (int) c_3(D);
bc_5 = D.2014_2 * D.2015_4;
D.2017_6 = (short int) bc_5;
D.2018_7 = (long long int) D.2017_6;
D.2016_9 = D.2018_7 + a_8(D);
return D.2016_9;
}
Here we have a multiply and accumulate done the long way. The 8-bit
inputs are widened to 32-bit, multiplied to give a 32-bit result (of
which only the lower 16-bits contain meaningful data), then truncated to
16-bits, and sign-extended up to 64-bits ready for the 64-bit addition.
This is slight contrived, perhaps, but not unlike the sort of thing that
might occur when you have inline functions and macros, and most
importantly - it is mathematically valid!
So, here's the output from my patched widen_mult pass:
foo (long long int a, signed char b, signed char c)
{
int bc;
long long int D.2018;
short int D.2017;
long long int D.2016;
int D.2015;
int D.2014;
<bb 2>:
D.2014_2 = (int) b_1(D);
D.2015_4 = (int) c_3(D);
bc_5 = b_1(D) w* c_3(D);
D.2017_6 = (short int) bc_5;
D.2018_7 = (long long int) D.2017_6;
D.2016_9 = WIDEN_MULT_PLUS_EXPR <b_1(D), c_3(D), a_8(D)>;
return D.2016_9;
}
As you can see, everything except the WIDEN_MULT_PLUS_EXPR statement is
now redundant. (Ideally, this would be removed now, but in fact it
doesn't get eliminated until the RTL into_cfglayout pass. This is not
new behaviour.)
My point is that it's possible to have at least two conversions to
examine. Is it possible to have more? I don't know, but once I'm dealing
with two I might as well deal with an arbitrary number.
Andrew
