https://llvm.org/bugs/show_bug.cgi?id=31412
Bug ID: 31412
Summary: SCEV unable to infer loop max bound for remainder
loops
Product: libraries
Version: trunk
Hardware: PC
OS: Linux
Status: NEW
Severity: normal
Priority: P
Component: Loop Optimizer
Assignee: unassignedb...@nondot.org
Reporter: mku...@google.com
CC: llvm-bugs@lists.llvm.org,
san...@playingwithpointers.com
Classification: Unclassified
Consider:
void foo0(int* in, int* out, unsigned k) {
k %= 4;
for (int i = 0; i < k; ++i) {
in[i] += out[i];
}
}
$ bin/clang -m32 -c -S -o - -O2 ~/llvm/temp/smallmax.cpp -emit-llvm | bin/opt
-analyze -scalar-evolution
[...]
Loop %for.body: backedge-taken count is (-1 + (zext i2 (trunc i32 %k to i2) to
i32))<nsw>
Loop %for.body: max backedge-taken count is -1
After talking to Sanjoy, it seems that the main issue is that
max-backedge-taken computation does not take the fact that "if the backedge is
taken, necessarily k > 0" into account.
Indeed:
void foo2(int* in, int* out, unsigned k) {
k %= 4;
k += 2;
for (int i = 0; i < k; ++i) {
in[i] += out[i];
}
}
$ bin/clang -m32 -c -S -o - -O2 ~/llvm/temp/smallmax.cpp -emit-llvm | bin/opt
-analyze -scalar-evolution
[...]
Loop %for.body: backedge-taken count is (1 + (zext i2 (trunc i32 %k to i2) to
i32))<nuw><nsw>
Loop %for.body: max backedge-taken count is 4
But I think this is not the only problem.
If it were, adding 1 to k should have been sufficient, but:
void foo1(int* in, int* out, unsigned k) {
k %= 4;
k += 1;
for (int i = 0; i < k; ++i) {
in[i] += out[i];
}
}
$ bin/clang -m32 -c -S -o - -O2 ~/llvm/temp/smallmax.cpp -emit-llvm | bin/opt
-analyze -scalar-evolution
[...]
Loop %for.body: backedge-taken count is (zext i2 (trunc i32 %k to i2) to i32)
Loop %for.body: max backedge-taken count is -1
--
You are receiving this mail because:
You are on the CC list for the bug.
_______________________________________________
llvm-bugs mailing list
llvm-bugs@lists.llvm.org
http://lists.llvm.org/cgi-bin/mailman/listinfo/llvm-bugs
