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 <roger@nextmovesoftware.com>
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.