GCC Bugzilla – Bug 32503
__builtin_powi - optimize for other exponents besides 2 and 0.5
Last modified: 2007-06-26 19:00:20 UTC
http://gcc.gnu.org/ml/gcc-patches/2007-06/msg01172.html (PR 32239):
"At the moment the vectorizer only vectorizes builtin_pow if the exponent is either 2 or 0.5. whereas if we expand constant exponents in the gfortran frontend (gfc_conv_cst_int_power) it vectorizes for other constant integer powers as well (the expansion by the frontend generates a number of calls to builtin_pow with exponent 2.0)."
Expected: The same optimization is done for multiple of 2 (and -2) - as it is currently in the Fortran FE. (If this has been implemented in the ME, please inform the Fortraners to remove the expansion there.)
Additionally: It would be great if there would be a variant for long integers
(8 byte integers) and long long (16 byte integers) as PR32239 could only use __builtin_powi for int4 and not for int8 or int16.
> Additionally: It would be great if there would be a variant for long integers
> (8 byte integers) and long long (16 byte integers) as PR32239 could only use
> __builtin_powi for int4 and not for int8 or int16.
Actually, gfortran uses also: pow(integer,integer) and pow(complex,integer), which could also profit from a middle/back-end support. (integer: 4, 8, 16 bytes; complex: single, double, long double precision).
Confirmed. I had done tree-level expansion of powi into add/mul sequences at
one time. But this had been rejected for some reason I cannot remember right now.
(In reply to comment #2)
> Confirmed. I had done tree-level expansion of powi into add/mul sequences at
> one time. But this had been rejected for some reason I cannot remember right
The middle-end is able to expand builtin_powi for constant integers. The problem with vectorization is that the vectorizer runs before the pow(i) expansion, and the vectorizer is only aware of exponenets 2 and 0.5. Expanding in the FE works around that problem. In fact, it seems the FE expander is lifted from the ME.