https://llvm.org/bugs/show_bug.cgi?id=26428
Bug ID: 26428
Summary: LICM: Ignore loop exits which don't exit on first
iteration when computing isGuaranteedToExecute
Product: libraries
Version: trunk
Hardware: PC
OS: Linux
Status: NEW
Severity: normal
Priority: P
Component: Loop Optimizer
Assignee: [email protected]
Reporter: [email protected]
CC: [email protected]
Classification: Unclassified
When computing the isGuaranteedToExecute predicate in LICM (used for satisfying
dereferenceability and memory model legality questions), we can ignore any
exiting blocks where the exit condition is known not to exit on the first
iteration.
A motivating example might be something like this:
sum = 0;
for (int i = 0; i < 100; i++)
if (i > 20) break;
// g_idx is a global and doesn't alias a
sum += a[g_idx]
}
We can hoist the load of g_idx out of the loop even if we don't know that g_idx
is dereferenceable since doing so can not change the location of a fault.
Specifically, we know that the loop exit can't be taken on the first iteration
since (0 > 20) == false.
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