https://llvm.org/bugs/show_bug.cgi?id=30429
Bug ID: 30429
Summary: Should recognize redundant comparisons with constant
offsets
Product: new-bugs
Version: trunk
Hardware: All
OS: All
Status: NEW
Keywords: code-quality
Severity: enhancement
Priority: P
Component: new bugs
Assignee: unassignedb...@nondot.org
Reporter: matti.niemenmaa+llvmb...@iki.fi
CC: llvm-bugs@lists.llvm.org
Classification: Unclassified
Consider the following pair of C functions that are both equivalent to "return
0":
int one(int i, int len) {
return i < len && i + 1 > len;
}
int two(int i, int len) {
return i + 1 < len && i + 2 > len;
}
They map to the following LLVM functions (simplified a bit):
define i32 @one(i32 %i, i32 %len) {
%1 = icmp slt i32 %i, %len
%2 = add nsw i32 %i, 1
%3 = icmp sgt i32 %2, %len
%4 = and i1 %1, %3
%5 = zext i1 %4 to i32
ret i32 %5
}
define i32 @two(i32 %i, i32 %len) {
%1 = add nsw i32 %i, 1
%2 = icmp slt i32 %1, %len
%3 = add nsw i32 %i, 2
%4 = icmp sgt i32 %3, %len
%5 = and i1 %2, %4
%6 = zext i1 %5 to i32
ret i32 %6
}
Now, opt -instcombine optimizes @one to "ret i32 0" but leaves @two untouched.
This is because InstCombineCompares.cpp has logic for "icmp sgt (X + 1), Y ->
icmp sge X, Y" which starts the chain of optimizations required in @one. I
considered extending this to "icmp sgt (X + C), Y -> icmp sge (X + (C - 1)), Y"
(and analogously for the other similar icmp optimizations) but this isn't
always a profitable transformation and seems oddly specific anyway. It's only
worth it here because we happen to have "icmp slt (X + (C - 1)), Y" nearby and
can combine the two.
Overall this kind of thing seems like something LLVM should be able to
optimize, but I'm not sure how it should go about doing it. I would imagine
cases like these to show up in e.g. array bounds checks on occasion.
This was with LLVM trunk r281814 and release version 3.9.0.
For what it's worth, GCC 6.2.1 can optimize both C functions to return 0, but
even it is stumped by the following:
int two_trickier(int i, int len) {
return i + 1 < len && i + 1 > len - 1;
}
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