https://gcc.gnu.org/bugzilla/show_bug.cgi?id=95862
--- Comment #4 from CVS Commits <cvs-commit at gcc dot gnu.org> --- The master branch has been updated by Jakub Jelinek <ja...@gcc.gnu.org>: https://gcc.gnu.org/g:049ce9d233e2d865dc81a5042b1c28ee21d1c9d8 commit r11-5366-g049ce9d233e2d865dc81a5042b1c28ee21d1c9d8 Author: Jakub Jelinek <ja...@redhat.com> Date: Wed Nov 25 15:42:38 2020 +0100 middle-end: __builtin_mul_overflow expansion improvements [PR95862] The following patch adds some improvements for __builtin_mul_overflow expansion. One optimization is for the u1 * u2 -> sr case, as documented we normally do: u1 * u2 -> sr res = (S) (u1 * u2) ovf = res < 0 || main_ovf (true) where main_ovf (true) stands for jump on unsigned multiplication overflow. If we know that the most significant bits of both operands are clear (such as when they are zero extended from something smaller), we can emit better coe by handling it like s1 * s2 -> sr, i.e. just jump on overflow after signed multiplication. Another two cases are s1 * s2 -> ur or s1 * u2 -> ur, if we know the minimum precision needed to encode all values of both arguments summed together is smaller or equal to destination precision (such as when the two arguments are sign (or zero) extended from half precision types, we know the overflows happen only iff one argument is negative and the other argument is positive (not zero), because even if both have maximum possible values, the maximum is still representable (e.g. for char * char -> unsigned short 0x7f * 0x7f = 0x3f01 and for char * unsigned char -> unsigned short 0x7f * 0xffU = 0x7e81) and as the result is unsigned, all negative results do overflow, but are also representable if we consider the result signed - all of them have the MSB set. So, it is more efficient to just do the normal multiplication in that case and compare the result considered as signed value against 0, if it is smaller, overflow happened. And the get_min_precision change is to improve the char to short handling, we have there in the IL _2 = (int) arg_1(D); promotion from C promotions from char or unsigned char arg, and the caller adds a NOP_EXPR cast to short or unsigned short. get_min_precision punts on the narrowing cast though, it handled only widening casts, but we can handle narrowing casts fine too, by recursing on the narrowing cast operands and using it only if it has in the end smaller minimal precision, which would duplicate the sign bits (or zero bits) to both the bits above the narrowing conversion and also at least one below that. 2020-10-25 Jakub Jelinek <ja...@redhat.com> PR rtl-optimization/95862 * internal-fn.c (get_min_precision): For narrowing conversion, recurse on the operand and if the operand precision is smaller than the current one, return that smaller precision. (expand_mul_overflow): For s1 * u2 -> ur and s1 * s2 -> ur cases if the sum of minimum precisions of both operands is smaller or equal to the result precision, just perform normal multiplication and set overflow to the sign bit of the multiplication result. For u1 * u2 -> sr if both arguments have the MSB known zero, use normal s1 * s2 -> sr expansion. * gcc.dg/builtin-artih-overflow-5.c: New test.
