On 11/14/2014 01:39 PM, Richard Henderson wrote:
On 11/13/2014 06:12 PM, Bastian Koppelmann wrote:
+ tcg_gen_ext_i32_i64(t3, r3);
+ tcg_gen_concat_i32_i64(t2, r2_low, r2_high);
+ /* extend the sign for r2 to high 64 bits */
+ tcg_gen_sari_i64(t4, t2, 63);
+ tcg_gen_ext_i32_i64(t1, r1);
+
+ tcg_gen_muls2_i64(t1, t3, t1, t3);
+ tcg_gen_add2_i64(t1, t3, t2, t4, t1, t3);
+
I don't believe that you need 128 bit arithemetic for multiply-accumulate,
either here or elsewhere (e.g. msub).
Looking at unsigned, the maximum result of the multiply is 2*(2^n-1), or 2^(2n)
- 2^(n+1). Which means that the accumulate with a 2^n-1 value cannot overflow
a double-word intermediate result.
Madd.u has the following signature 64 + (32 * 32) --> 64, as far as I
read the documentation, and would result as you described in a max
result of 2^(2n) - 2^(n+1) for the multiplication, but it would
accumulate with 2^(2n) -1, which can definitly overflow, with n = 32.
However for signed multiply accumulate I don't need 128 bit arithmetic,
because only the add/sub operation of those two can overflow. Thanks for
the tip!
Cheers,
Bastian
