From d863ba23fb16122bb0547b0c678173be0d98f43c Mon Sep 17 00:00:00 2001 From: Roger Sayle Date: Thu, 19 May 2022 17:54:38 +0100 Subject: PR middle-end/98865: Expand X*Y as X&-Y when Y is [0,1]. The patch is a revised solution for PR middle-end/98865 incorporating the feedback/suggestions from Richard Biener's review here: https://gcc.gnu.org/pipermail/gcc-patches/2022-May/593928.html Most significantly, this patch now performs the transformation/optimization during RTL expansion, where the target's rtx_costs can be used to determine whether the original multiplication (that may potentially be implemented by a shift or lea) is cheaper than a negation and a bit-wise and. Previously the expression (x>>63)*y would be compiled with -O2 as shrq $63, %rdi movq %rdi, %rax imulq %rsi, %rax but with this patch now produces: sarq $63, %rdi movq %rdi, %rax andq %rsi, %rax Likewise the expression (x>>63)*135 [that appears in a hot-spot of the Botan AES-128 benchmark] was previously: shrq $63, %rdi leaq (%rdi,%rdi,8), %rdx movq %rdx, %rax salq $4, %rax subq %rdx, %rax now becomes: movq %rdi, %rax sarq $63, %rax andl $135, %eax 2022-05-19 Roger Sayle gcc/ChangeLog PR middle-end/98865 * expr.cc (expand_expr_real_2) [MULT_EXPR]: Expand X*Y as X&Y when both X and Y are [0, 1], X*Y as X&-Y when Y is [0,1] and likewise X*Y as -X&Y when X is [0,1] using tree_nonzero_bits. gcc/testsuite/ChangeLog PR middle-end/98865 * gcc.target/i386/pr98865.c: New test case. --- gcc/expr.cc | 32 ++++++++++++++++++++++++++++++++ 1 file changed, 32 insertions(+) (limited to 'gcc/expr.cc') diff --git a/gcc/expr.cc b/gcc/expr.cc index 1806091..7197996 100644 --- a/gcc/expr.cc +++ b/gcc/expr.cc @@ -9541,6 +9541,38 @@ expand_expr_real_2 (sepops ops, rtx target, machine_mode tmode, } expand_operands (treeop0, treeop1, subtarget, &op0, &op1, EXPAND_NORMAL); + + /* Expand X*Y as X&-Y when Y must be zero or one. */ + if (SCALAR_INT_MODE_P (mode)) + { + bool bit0_p = tree_nonzero_bits (treeop0) == 1; + bool bit1_p = tree_nonzero_bits (treeop1) == 1; + + /* Expand X*Y as X&Y when both X and Y must be zero or one. */ + if (bit0_p && bit1_p) + return REDUCE_BIT_FIELD (expand_and (mode, op0, op1, target)); + + if (bit0_p || bit1_p) + { + bool speed = optimize_insn_for_speed_p (); + int cost = add_cost (speed, mode) + neg_cost (speed, mode); + struct algorithm algorithm; + enum mult_variant variant; + if (CONST_INT_P (op1) + ? !choose_mult_variant (mode, INTVAL (op1), + &algorithm, &variant, cost) + : cost < mul_cost (speed, mode)) + { + target = bit0_p ? expand_and (mode, negate_rtx (mode, op0), + op1, target) + : expand_and (mode, op0, + negate_rtx (mode, op1), + target); + return REDUCE_BIT_FIELD (target); + } + } + } + return REDUCE_BIT_FIELD (expand_mult (mode, op0, op1, target, unsignedp)); case TRUNC_MOD_EXPR: -- cgit v1.1