An issue with a backend patch I've been investigating has revealed
a missed optimization opportunity during GCC's vector lowering pass.
An unrecognized insn for "(set (reg:SI) (not:SI (const_int 0))"
revealed that not only was my expander not expecting a NOT with
a constant operand, but also that veclower was producing the
dubious tree expression ~0.
The attached patch replaces a call to gimple_build_assign with a
call to either gimplify_build1 or gimplify_build2 depending upon
whether the operation takes one or two operands. The net effect
is that where GCC previously produced the following optimized
gimple for testsuite/c-c++common/Wunused-var-16.c (notice the ~0
and the "& 0"):
void foo ()
{
V x;
V y;
vector(16) unsigned char _1;
unsigned char _7;
unsigned char _8;
y_2 = { 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0 };
x_3 = { 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0 };
_7 = ~0;
_1 = {_7, _7, _7, _7, _7, _7, _7, _7, _7, _7, _7, _7, _7, _7, _7, _7};
_8 = 0 & _7;
y_4 = {_8, _8, _8, _8, _8, _8, _8, _8, _8, _8, _8, _8, _8, _8, _8, _8};
v = y_4;
return;
}
With this patch we now generate:
void foo ()
{
V x;
V y;
vector(16) unsigned char _1;
y_2 = { 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0 };
x_3 = { 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0 };
_1 = { 255, 255, 255, 255, 255, 255, 255, 255, 255, 255, 255, 255, 255,
255, 255, 255 };
y_4 = { 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0 };
v = y_4;
return;
}
This patch has been tested on x86_64-pc-linux-gnu with "make bootstrap"
and "make -k check".
Ok for mainline?
2021-08-18 Roger Sayle <ro...@nextmovesoftware.com>
gcc/ChangeLog
* tree-vect-generic.c (expand_vector_operations_1): Use either
gimplify_build1 or gimplify_build2 instead of gimple_build_assign
when constructing scalar splat expressions.
gcc/testsuite/ChangeLog
* c-c++common/Wununsed-var-16.c: Add extra check for that ~0
is optimized away.
