https://bugs.llvm.org/show_bug.cgi?id=42389
Bug ID: 42389
Summary: InstCombine: lshr + "sext" => ashr canonicalization
Product: libraries
Version: trunk
Hardware: PC
OS: Linux
Status: NEW
Severity: enhancement
Priority: P
Component: Scalar Optimizations
Assignee: unassignedb...@nondot.org
Reporter: lebedev...@gmail.com
CC: llvm-bugs@lists.llvm.org
I have not triaged yet what folds are missing, but this gigantic fold is
missing:
https://godbolt.org/z/E9-txU
int bad(unsigned input, unsigned len) {
unsigned diff = input >> (32 - len);
// If the first bit is 1 we need to turn this into a negative number
if (diff >> (len - 1))
diff -= (1 << len);
return diff;
}
int good(unsigned input, unsigned len) {
return int(input) >> (32 - len);
}
https://rise4fun.com/Alive/l3k
Name: ashr
%o3 = sub i32 32, %nbits
%o4 = lshr i32 %x, %o3
%o5 = add i32 %nbits, -1
%o6 = lshr i32 %o4, %o5
%o7 = icmp eq i32 %o6, 0
%o8 = shl i32 1, %nbits
%o9 = select i1 %o7, i32 0, i32 %o8
%r = sub i32 %o4, %o9
=>
%n0 = sub i32 32, %nbits
%r = ashr i32 %x, %n0
I'm seeing this in real code.
--
You are receiving this mail because:
You are on the CC list for the bug._______________________________________________
llvm-bugs mailing list
llvm-bugs@lists.llvm.org
https://lists.llvm.org/cgi-bin/mailman/listinfo/llvm-bugs
