https://bugs.llvm.org/show_bug.cgi?id=35102
Bug ID: 35102
Summary: InstCombiner::foldAllocaCmp does not check whether
icmp is not in a loop
Product: new-bugs
Version: trunk
Hardware: PC
OS: Linux
Status: NEW
Severity: normal
Priority: P
Component: new bugs
Assignee: unassignedb...@nondot.org
Reporter: juneyoung....@sf.snu.ac.kr
CC: llvm-bugs@lists.llvm.org
Created attachment 19352
--> https://bugs.llvm.org/attachment.cgi?id=19352&action=edit
cpp code that removes for loop away
Compiling this code with -O3 generates following assembly code:
-- unreachable.cpp --
#include <stdint.h>
void unreachable();
static void compare_to_all(uintptr_t i) {
uintptr_t cur = 0;
while(true) {
if ((int*)i == (int*)cur) return;
if (cur == UINTPTR_MAX) break;
++cur;
};
unreachable();
}
void test() {
int i = 1;
compare_to_all((uintptr_t)&i);
}
--
-- unreachable.s --
test(): # @test()
jmp unreachable() # TAILCALL
--
InstCombiner::foldAllocaCmp folds `(int*)i == (int*)cur` into false after
checking that there exists only one icmp instruction which uses alloca i.
If the purpose of this optimization was to fold single pointer equality
comparison into false, syntactically checking the number of icmp isn't enough.
It should also check whether the icmp is not in a loop. In following code
(test2.c), this optimization folds the pointer comparison in foo(), but not in
gee().
-- test2.c --
void foo(int* p) {
int i;
for (int j = 0; j < 2; j++) {
if (&i == p+j) {
printf("ok\n");
}
}
}
void gee(int* p) {
int i;
if (&i == p+0) {
printf("ok\n");
}
if (&i == p+1) {
printf("ok\n");
}
}
--
unreachable.cpp written by Ralf Jung, test2.c written by Gil Hur
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