My sincere apologies for the breakage. My recent patch to fold bswapN(x)>>C where the constant C was large enough that the result only contains bits from the low byte, and can therefore avoid the byte swap contains a minor logic error. The pattern contains a convert? allowing an extension to occur between the bswap and the shift. The logic is correct if there's no extension, or the extension has the same sign as the shift, but I'd mistakenly convinced myself that these couldn't have different signedness.
(T)bswap16(x)>>12 is (T)((unsigned char)x>>4) or (T)((signed char)x>>4). The bug is that for zero-extensions to signed type T, we need to use the unsigned char variant [the signedness of the byte shift is not (always) the same as the signedness of T and the original shift]. Then because I'm now paranoid, I've also added a clause to handle the hypothetical (but in practice impossible) sign-extension to an unsigned type T, which can implemented as (T)(x<<8)>>12. This patch has been tested on x86_64-pc-linux-gnu with a "make bootstrap" and "make -k check" with no new failures, and a new testcase to confirm it fixes the regression. Ok for mainline? 2021-07-11 Roger Sayle <ro...@nextmovesoftware.com> gcc/ChangeLog PR tree-optimization/101403 * gcc/match.pd ((T)bswap(X)>>C): Correctly handle cases where signedness of the shift is not the same as the signedness of the type extension. gcc/testsuite/ChangeLog PR tree-optimization/101403 * gcc.dg/pr101403.c: New test case. Sorry again, Roger -- Roger Sayle NextMove Software Cambridge, UK
diff --git a/gcc/match.pd b/gcc/match.pd index 30680d4..beb8d27 100644 --- a/gcc/match.pd +++ b/gcc/match.pd @@ -3659,19 +3659,31 @@ DEFINE_INT_AND_FLOAT_ROUND_FN (RINT) { unsigned HOST_WIDE_INT prec = TYPE_PRECISION (TREE_TYPE (@2)); unsigned HOST_WIDE_INT bits = tree_to_uhwi (@1); + /* If the bswap was extended before the original shift, this + byte (shift) has the sign of the extension, not the sign of + the original shift. */ + tree st = TYPE_PRECISION (type) > prec ? TREE_TYPE (@2) : type; } - (if (bits + 8 == prec) - (if (TYPE_UNSIGNED (type)) - (convert (convert:unsigned_char_type_node @0)) - (convert (convert:signed_char_type_node @0))) - (if (bits < prec && bits + 8 > prec) - (with - { - tree nst = build_int_cst (integer_type_node, bits & 7); - tree bt = TYPE_UNSIGNED (type) ? unsigned_char_type_node - : signed_char_type_node; - } - (convert (rshift:bt (convert:bt @0) {nst;})))))))) + /* Special case: logical right shift of sign-extended bswap. + (unsigned)(short)bswap16(x)>>12 is (unsigned)((short)x<<8)>>12. */ + (if (TYPE_PRECISION (type) > prec + && !TYPE_UNSIGNED (TREE_TYPE (@2)) + && TYPE_UNSIGNED (type) + && bits < prec && bits + 8 >= prec) + (with { tree nst = build_int_cst (integer_type_node, prec - 8); } + (rshift (convert (lshift:st (convert:st @0) {nst;})) @1)) + (if (bits + 8 == prec) + (if (TYPE_UNSIGNED (st)) + (convert (convert:unsigned_char_type_node @0)) + (convert (convert:signed_char_type_node @0))) + (if (bits < prec && bits + 8 > prec) + (with + { + tree nst = build_int_cst (integer_type_node, bits & 7); + tree bt = TYPE_UNSIGNED (st) ? unsigned_char_type_node + : signed_char_type_node; + } + (convert (rshift:bt (convert:bt @0) {nst;}))))))))) /* bswap(x) & C1 can sometimes be simplified to (x >> C2) & C1. */ (simplify (bit_and (convert? (bswap@2 @0)) INTEGER_CST@1)
/* { dg-do run } */ /* { dg-options "-O2" } */ unsigned int foo (unsigned int a) { unsigned int u; unsigned short b = __builtin_bswap16 (a); return b >> (u, 12); } int main (void) { unsigned int x = foo (0x80); if (x != 0x0008) __builtin_abort (); return 0; }