This patch resolves PR tree-optimization/71343, a missed-optimization enhancement request where GCC fails to see that (a<<2)+(b<<2) == a*4+b*4. This requires two related (sets of) optimizations to be added to match.pd.
The first is that (X<<C) op (Y<<C) can be simplified to (X op Y) << C, for many binary operators, including AND, IOR, XOR, and (if overflow isn't an issue) PLUS and MINUS. Likewise, the right shifts (both logical and arithmetic) and bit-wise logical operators can be simplified in a similar fashion. These all reduce the number of GIMPLE binary operations from 3 to 2, by combining/eliminating a shift operation. The second optimization reflects that the middle-end doesn't impose a canonical form on multiplications by powers of two, vs. left shifts, instead leaving these operations as specified by the programmer unless there's a good reason to change them. Hence, GIMPLE code may contain the expressions "X * 8" and "X << 3" even though these represent the same value/computation. The tweak to match.pd is that comparison operations whose operands are equivalent non-canonical expressions can be taught their equivalence. Hence "(X * 8) == (X << 3)" will always evaluate to true, and "(X<<2) > 4*X" will always evaluate to false. This patch has been tested on x86_64-pc-linux-gnu with make bootstrap and make -k check, both with and without --target_board=unix{-m32}, with no new failures. Ok for mainline? 2022-08-08 Roger Sayle <ro...@nextmovesoftware.com> gcc/ChangeLog PR tree-optimization/71343 * match.pd (op (lshift @0 @1) (lshift @2 @1)): Optimize the expression (X<<C) + (Y<<C) to (X+Y)<<C for multiple operators. (op (rshift @0 @1) (rshift @2 @1)): Likwise, simplify (X>>C)^(Y>>C) to (X^Y)>>C for binary logical operators, AND, IOR and XOR. (cmp:c (lshift @0) (mult @1)): Optimize comparisons between shifts by integer constants and multiplications by powers of 2. gcc/testsuite/ChangeLog PR tree-optimization/71343 * gcc.dg/pr71343-1.c: New test case. * gcc.dg/pr71343-2.c: Likewise. Thanks in advance, Roger --
diff --git a/gcc/match.pd b/gcc/match.pd index f82f94a..d6b9cfb 100644 --- a/gcc/match.pd +++ b/gcc/match.pd @@ -982,6 +982,35 @@ DEFINE_INT_AND_FLOAT_ROUND_FN (RINT) && tree_nop_conversion_p (type, TREE_TYPE (@1))) (lshift @0 @2))) +/* Shifts by constants distribute over several binary operations, + hence (X << C) + (Y << C) can be simplified to (X + Y) << C. */ +(for op (plus minus) + (simplify + (op (lshift:s @0 INTEGER_CST@1) (lshift:s @2 INTEGER_CST@1)) + (if (INTEGRAL_TYPE_P (type) + && TYPE_OVERFLOW_WRAPS (type) + && !TYPE_SATURATING (type) + && tree_fits_shwi_p (@1) + && tree_to_shwi (@1) > 0 + && tree_to_shwi (@1) < TYPE_PRECISION (type)) + (lshift (op @0 @2) @1)))) + +(for op (bit_and bit_ior bit_xor) + (simplify + (op (lshift:s @0 INTEGER_CST@1) (lshift:s @2 INTEGER_CST@1)) + (if (INTEGRAL_TYPE_P (type) + && tree_fits_shwi_p (@1) + && tree_to_shwi (@1) > 0 + && tree_to_shwi (@1) < TYPE_PRECISION (type)) + (lshift (op @0 @2) @1))) + (simplify + (op (rshift:s @0 INTEGER_CST@1) (rshift:s @2 INTEGER_CST@1)) + (if (INTEGRAL_TYPE_P (type) + && tree_fits_shwi_p (@1) + && tree_to_shwi (@1) > 0 + && tree_to_shwi (@1) < TYPE_PRECISION (type)) + (rshift (op @0 @2) @1)))) + /* Fold (1 << (C - x)) where C = precision(type) - 1 into ((1 << C) >> x). */ (simplify @@ -2241,6 +2270,28 @@ DEFINE_INT_AND_FLOAT_ROUND_FN (RINT) (if (TREE_INT_CST_LOW (@1) & 1) { constant_boolean_node (cmp == NE_EXPR, type); }))) +/* Some tree expressions are intentionally non-canonical. + We handle the comparison of the equivalent forms here. */ +(for cmp (eq le ge) + (simplify + (cmp:c (lshift @0 INTEGER_CST@1) (mult @0 integer_pow2p@2)) + (if (INTEGRAL_TYPE_P (TREE_TYPE (@0)) + && tree_fits_shwi_p (@1) + && tree_to_shwi (@1) > 0 + && tree_to_shwi (@1) < TYPE_PRECISION (TREE_TYPE (@0)) + && wi::to_wide (@1) == wi::exact_log2 (wi::to_wide (@2))) + { constant_boolean_node (true, type); }))) + +(for cmp (ne lt gt) + (simplify + (cmp:c (lshift @0 INTEGER_CST@1) (mult @0 integer_pow2p@2)) + (if (INTEGRAL_TYPE_P (TREE_TYPE (@0)) + && tree_fits_shwi_p (@1) + && tree_to_shwi (@1) > 0 + && tree_to_shwi (@1) < TYPE_PRECISION (TREE_TYPE (@0)) + && wi::to_wide (@1) == wi::exact_log2 (wi::to_wide (@2))) + { constant_boolean_node (false, type); }))) + /* Arguments on which one can call get_nonzero_bits to get the bits possibly set. */ (match with_possible_nonzero_bits diff --git a/gcc/testsuite/gcc.dg/pr71343-1.c b/gcc/testsuite/gcc.dg/pr71343-1.c new file mode 100644 index 0000000..146f5fc --- /dev/null +++ b/gcc/testsuite/gcc.dg/pr71343-1.c @@ -0,0 +1,56 @@ +/* { dg-do compile } */ +/* { dg-options "-O2 -fdump-tree-optimized" } */ + +unsigned int foo_plus(unsigned int a, unsigned int b) +{ + return (a << 2) + (b << 2); +} + +unsigned int foo_and(unsigned int a, unsigned int b) +{ + return (a << 2) & (b << 2); +} + +unsigned int foo_ior(unsigned int a, unsigned int b) +{ + return (a << 2) | (b << 2); +} + +unsigned int foo_xor(unsigned int a, unsigned int b) +{ + return (a << 2) ^ (b << 2); +} + +unsigned int bar_and(unsigned int a, unsigned int b) +{ + return (a >> 2) & (b >> 2); +} + +unsigned int bar_ior(unsigned int a, unsigned int b) +{ + return (a >> 2) | (b >> 2); +} + +unsigned int bar_xor(unsigned int a, unsigned int b) +{ + return (a >> 2) ^ (b >> 2); +} + +int baz_and(int a, int b) +{ + return (a >> 2) & (b >> 2); +} + +int baz_ior(int a, int b) +{ + return (a >> 2) | (b >> 2); +} + +int baz_xor(int a, int b) +{ + return (a >> 2) ^ (b >> 2); +} + +/* { dg-final { scan-tree-dump-times " << " 4 "optimized" } } */ +/* { dg-final { scan-tree-dump-times " >> " 6 "optimized" } } */ + diff --git a/gcc/testsuite/gcc.dg/pr71343-2.c b/gcc/testsuite/gcc.dg/pr71343-2.c new file mode 100644 index 0000000..11800a9 --- /dev/null +++ b/gcc/testsuite/gcc.dg/pr71343-2.c @@ -0,0 +1,34 @@ +/* { dg-do compile } */ +/* { dg-options "-O2 -fdump-tree-optimized" } */ + +unsigned int test1(unsigned int a , unsigned int b) +{ + return (a << 2) + (b << 2) == a * 4 + b * 4; +} + +unsigned int test2(unsigned int a , unsigned int b) +{ + return (a << 2) + (b << 2) == (a + b) << 2; +} + +unsigned int test3(unsigned int a , unsigned int b) +{ + return a * 4 + b * 4 == (a + b) * 4; +} + +unsigned int test4(unsigned int a , unsigned int b) +{ + return (a + b) << 2 == (a + b) * 4; +} + +unsigned int test5(unsigned int a , unsigned int b) +{ + return (a << 2) + (b << 2) == (a + b) * 4; +} + +unsigned int test6(unsigned int a , unsigned int b) +{ + return (a + b) << 2 == a * 4 + b * 4; +} + +/* { dg-final { scan-tree-dump-times "return 1" 6 "optimized" } } */