https://bugs.llvm.org/show_bug.cgi?id=49014
Bug ID: 49014
Summary: IRCE wouldn't work when trip count is unsigned and br
condition is equality.
Product: libraries
Version: 11.0
Hardware: PC
OS: Linux
Status: NEW
Severity: enhancement
Priority: P
Component: Loop Optimizer
Assignee: unassignedb...@nondot.org
Reporter: jie.he...@gmail.com
CC: llvm-bugs@lists.llvm.org
please refer to Bug 49012 for the story.
this time, I made a little change in the previous code. I updated the exit
conditon to "NE" from "ULT", like below:
void testIRCE(unsigned int * buf, unsigned int len, unsigned int
iteration_count) {
if (iteration_count > 0) {
unsigned int i = 0;
do {
if (i >= len) { // range check
printf("overflow\n");
return;
}
buf[i] = i;
i ++;
} while (i != iteration_count);
}
}
from C code, we know iteration_count is always an non-negative value (it's an
unsigned int and I add a condition guard outside the loop), and the loop will
exit as expected. but IRCE doesn't work because it thinks iteration_count will
overflow. see the code in function LoopStructure::parseLoopStructure():
if (ICI->isEquality() && !HasNoSignedWrap(IndVarBase)) {
FailureReason = "LHS in icmp needs nsw for equality predicates";
return None;
}
then I skip the check and go to the next. soon，I meet another check when try to
replace NE to ULT, it seems still can't prove iteration_count is non-negative.
see the code in function LoopStructure::parseLoopStructure():
if (isKnownNonNegativeInLoop(IndVarStart, &L, SE) &&
isKnownNonNegativeInLoop(RightSCEV, &L, SE))
Pred = ICmpInst::ICMP_ULT;
else
Pred = ICmpInst::ICMP_SLT;
the function isKnownNonNegativeInLoop is intended to prove iteration_count SGE
0. from C code, iteration_count is an unsigned int and "> 0", but at LLVM IR
code, it's hard to prove iteration_count SGE 0, LLVM IR doesn't care if it's an
unsigned int. I don't know if there is better way to let the compiler knows
iteration_count is great than 0.
finally, I have to change the type of iteration_count from unsigned to signed,
the optimization IRCE works.
I don't know if there is better idea to let the optimzation
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