GCC modified for the FreeChainXenon project
96146e61cd
The easiest way to motivate these additions to match.pd is with the
following example:
unsigned int foo(unsigned char i) {
return i | (i<<8) | (i<<16) | (i<<24);
}
which mainline with -O2 on x86_64 currently generates:
foo: movzbl %dil, %edi
movl %edi, %eax
movl %edi, %edx
sall $8, %eax
sall $16, %edx
orl %edx, %eax
orl %edi, %eax
sall $24, %edi
orl %edi, %eax
ret
but with this patch now becomes:
foo: movzbl %dil, %eax
imull $16843009, %eax, %eax
ret
Interestingly, this transformation is already applied when using
addition, allowing synth_mult to select an optimal sequence, but
not when using the equivalent bit-wise ior or xor operators.
The solution is to use tree_nonzero_bits to check that the
potentially non-zero bits of each operand don't overlap, which
ensures that BIT_IOR_EXPR and BIT_XOR_EXPR produce the same
results as PLUS_EXPR, which effectively generalizes the old
fold_plusminus_mult_expr. Technically, the transformation
is to canonicalize (X*C1)|(X*C2) and (X*C1)^(X*C2) to
X*(C1+C2) where X and X<<C are considered special cases.
2021-08-04 Roger Sayle <roger@nextmovesoftware.com>
Marc Glisse <marc.glisse@inria.fr>
gcc/ChangeLog
* match.pd (bit_ior, bit_xor): Canonicalize (X*C1)|(X*C2) and
(X*C1)^(X*C2) as X*(C1+C2), and related variants, using
tree_nonzero_bits to ensure that operands are bit-wise disjoint.
gcc/testsuite/ChangeLog
* gcc.dg/fold-ior-4.c: New test.
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