https://llvm.org/bugs/show_bug.cgi?id=23863
Bug ID: 23863
Summary: DAGCombiner missing byte swap idioms
Product: libraries
Version: trunk
Hardware: PC
OS: Linux
Status: NEW
Severity: normal
Priority: P
Component: Scalar Optimizations
Assignee: [email protected]
Reporter: [email protected]
CC: [email protected]
Classification: Unclassified
LLVM is missing an opportunity to reduce the following byte-swap routine into
(when available) target-specific byte swap routines:
uint32_t bswap(uint32_t x)
{
x = (x & 0x0000FFFF) << 16 | (x & 0xFFFF0000) >> 16;
x = (x & 0x00FF00FF) << 8 | (x & 0xFF00FF00) >> 8;
return x;
}
Also noticed this one mentioned in lib/Target/README.txt that I'll add here,
unsigned long reverse(unsigned v) {
unsigned t;
t = v ^ ((v << 16) | (v >> 16));
t &= ~0xff0000;
v = (v << 24) | (v >> 8);
return v ^ (t >> 8);
}
DAGCombiner should be taught about the first one at least, since GCC does
recognise that idiom. Whether LLVM should ever recognise the second one is left
to taste.
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