While looking into the possibility of introducing setmemM patterns on RISC-V to undo the transformation from loops of word writes into memset, I was disappointed to find out that get_nonzero_bits would take into account the range of the length passed to memset, but not the trivially-available observation that this length was a multiple of the word size. This knowledge, if passed on to setmemM, could enable setmemM to output more efficient code.
In the end, I did not introduce a setmemM pattern, nor the machinery to pass the ctz of the length on to it along with other useful information, but I figured this small improvement to nonzero_bits could still improve code generation elsewhere. https://gcc.gnu.org/pipermail/gcc-patches/2021-January/564341.html Regstrapped on x86_64-linux-gnu. No analysis of codegen impact yet. Does this seem worth pursuing, presumably for stage1? for gcc/ChangeLog * tree-ssanames.c (get_nonzero_bits): Zero out low bits of integral types, when a MULT_EXPR INTEGER_CST operand ensures the result will be a multiple of a power of two. --- gcc/tree-ssanames.c | 23 +++++++++++++++++++++-- 1 file changed, 21 insertions(+), 2 deletions(-) diff --git a/gcc/tree-ssanames.c b/gcc/tree-ssanames.c index 51a26d2fce1c2..c4b5bf2a4999a 100644 --- a/gcc/tree-ssanames.c +++ b/gcc/tree-ssanames.c @@ -546,10 +546,29 @@ get_nonzero_bits (const_tree name) } range_info_def *ri = SSA_NAME_RANGE_INFO (name); + wide_int ret; if (!ri) - return wi::shwi (-1, precision); + ret = wi::shwi (-1, precision); + else + ret = ri->get_nonzero_bits (); + + /* If NAME is defined as a multiple of a constant C, we know the ctz(C) low + bits are zero. ??? Should we handle LSHIFT_EXPR too? Non-constants, + e.g. the minimum shift count, and ctz from both MULT_EXPR operands? That + could make for deep recursion. */ + if (INTEGRAL_TYPE_P (TREE_TYPE (name)) + && SSA_NAME_DEF_STMT (name) + && is_gimple_assign (SSA_NAME_DEF_STMT (name)) + && gimple_assign_rhs_code (SSA_NAME_DEF_STMT (name)) == MULT_EXPR + && TREE_CODE (gimple_assign_rhs2 (SSA_NAME_DEF_STMT (name))) == INTEGER_CST) + { + unsigned HOST_WIDE_INT bits + = tree_ctz (gimple_assign_rhs2 (SSA_NAME_DEF_STMT (name))); + wide_int mask = wi::shwi (-1, precision) << bits; + ret &= mask; + } - return ri->get_nonzero_bits (); + return ret; } /* Return TRUE is OP, an SSA_NAME has a range of values [0..1], false -- Alexandre Oliva, happy hacker https://FSFLA.org/blogs/lxo/ Free Software Activist GNU Toolchain Engineer Vim, Vi, Voltei pro Emacs -- GNUlius Caesar
