https://gcc.gnu.org/bugzilla/show_bug.cgi?id=108874
Bug ID: 108874
Summary: [10/11/12/13 Regression] Missing bswap detection
Product: gcc
Version: unknown
Status: UNCONFIRMED
Keywords: missed-optimization
Severity: normal
Priority: P3
Component: tree-optimization
Assignee: unassigned at gcc dot gnu.org
Reporter: ktkachov at gcc dot gnu.org
Target Milestone: ---
If we look at the arm testcases in gcc.target/arm/rev16.c
typedef unsigned int __u32;
__u32
__rev16_32_alt (__u32 x)
{
return (((__u32)(x) & (__u32)0xff00ff00UL) >> 8)
| (((__u32)(x) & (__u32)0x00ff00ffUL) << 8);
}
__u32
__rev16_32 (__u32 x)
{
return (((__u32)(x) & (__u32)0x00ff00ffUL) << 8)
| (((__u32)(x) & (__u32)0xff00ff00UL) >> 8);
}
we should be able to generate rev16 instructions for aarch64 (and arm) i.e.
recognise a __builtin_bswap16 essentially.
GCC fails to do so and generates:
__rev16_32_alt:
lsr w1, w0, 8
lsl w0, w0, 8
and w1, w1, 16711935
and w0, w0, -16711936
orr w0, w1, w0
ret
__rev16_32:
lsl w1, w0, 8
lsr w0, w0, 8
and w1, w1, -16711936
and w0, w0, 16711935
orr w0, w1, w0
ret
whereas clang manages to recognise it all into:
__rev16_32_alt: // @__rev16_32_alt
rev16 w0, w0
ret
__rev16_32: // @__rev16_32
rev16 w0, w0
ret
does the bswap pass need some tweaking perhaps?
Looks like this worked fine with GCC 5 but broke in the GCC 6 timeframe so
marking as a regression
