https://bugs.llvm.org/show_bug.cgi?id=43564
Bug ID: 43564
Summary: [InstCombine] Failure to recognize sign bit test if
the high bit extract is followed by trunc
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
https://godbolt.org/z/hzDvD_
Given
int bad(unsigned input, unsigned len) {
unsigned diff = input & ((1 << len)-1);
// If the first bit is 1 we need to turn this into a negative number
if (diff >> (len - 1))
diff -= (1 << len);
return diff;
}
we get
%5 = icmp slt i32 %0, 0
But for
int bad2(unsigned long input, unsigned len) {
unsigned diff = input >> (64 - 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;
}
we no longer recognize it, and remain with
%7 = add i32 %1, -1
%8 = lshr i32 %6, %7
%9 = icmp eq i32 %8, 0
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