On 10/14/23 17:32, Roger Sayle wrote:
This patch improves the initial RTL expanded for double word shifts
on architectures with conditional moves, so that later passes don't
need to clean-up unnecessary and/or unused instructions.
Consider the general case, x << y, which is expanded well as:
t1 = y & 32;
t2 = 0;
t3 = x_lo >> 1;
t4 = y ^ ~0;
t5 = t3 >> t4;
tmp_hi = x_hi << y;
tmp_hi |= t5;
tmp_lo = x_lo << y;
out_hi = t1 ? tmp_lo : tmp_hi;
out_lo = t1 ? t2 : tmp_lo;
which is nearly optimal, the only thing that can be improved is
that using a unary NOT operation "t4 = ~y" is better than XOR
with -1, on targets that support it. [Note the one_cmpl_optab
expander didn't fall back to XOR when this code was originally
written, but has been improved since].
Now consider the relatively common idiom of 1LL << y, which
currently produces the RTL equivalent of:
t1 = y & 32;
t2 = 0;
t3 = 1 >> 1;
t4 = y ^ ~0;
t5 = t3 >> t4;
tmp_hi = 0 << y;
tmp_hi |= t5;
tmp_lo = 1 << y;
out_hi = t1 ? tmp_lo : tmp_hi;
out_lo = t1 ? t2 : tmp_lo;
Notice here that t3 is always zero, so the assignment of t5
is a variable shift of zero, which expands to a loop on many
smaller targets, a similar shift by zero in the first tmp_hi
assignment (another loop), that the value of t4 is no longer
required (as t3 is zero), and that the ultimate value of tmp_hi
is always zero.
Fortunately, for many (but perhaps not all) targets this mess
gets cleaned up by later optimization passes. However, this
patch avoids generating unnecessary RTL at expand time, by
calling simplify_expand_binop instead of expand_binop, and
avoiding generating dead or unnecessary code when intermediate
values are known to be zero. For the 1LL << y test case above,
we now generate:
t1 = y & 32;
t2 = 0;
tmp_hi = 0;
tmp_lo = 1 << y;
out_hi = t1 ? tmp_lo : tmp_hi;
out_lo = t1 ? t2 : tmp_lo;
On arc-elf, for example, there are 18 RTL INSN_P instructions
generated by expand before this patch, but only 12 with this patch
(improving both compile-time and memory usage).
This patch has been tested on x86_64-pc-linux-gnu with make bootstrap
and make -k check, both with and without --target_board=unix{-m32}
with no new failures. Ok for mainline?
2023-10-15 Roger Sayle <ro...@nextmovesoftware.com>
gcc/ChangeLog
* optabs.cc (expand_subword_shift): Call simplify_expand_binop
instead of expand_binop. Optimize cases (i.e. avoid generating
RTL) when CARRIES or INTO_INPUT is zero. Use one_cmpl_optab
(i.e. NOT) instead of xor_optab with ~0 to calculate ~OP1.
OK.
FWIW H8 is another one of the targets where variable shifts have to be
implemented as a loop.
Jeff
