http://llvm.org/bugs/show_bug.cgi?id=10377
Summary: Missed optimizations with indirectbr terminators
Product: new-bugs
Version: 2.9
Platform: PC
OS/Version: Linux
Status: NEW
Severity: normal
Priority: P
Component: new bugs
AssignedTo: [email protected]
ReportedBy: [email protected]
CC: [email protected]
Consider this IR fragment produced by opt -O3:
> %7:
> %8 = phi i8* [ blockaddress(@0, %19), %19 ], [ %12, %11 ]
> %9 = phi i32 [ %20, %19 ], [ 0, %11 ]
> indirectbr i8* %8, [label %4, label %19]
>
> %19:
> %20 = add nsw i32 %9, 1
> %21 = icmp eq i32 %9, 9999
> br i1 %21, label %16, label %7
the br in %19 should be optimized to branch directly to itself rather than
going back to %7 (note that the arg %8 to the indirectbr will always be the
address of %19 when coming from %19). While this might or might not be so big
of a problem per-se, notice how %19 is a loop body that simply adds 1 to the
same variable at each iteration. Because the br is still pointing to %7 instead
of %18, other passes are not able to recognize that the loop has a constant
iteration count and could therefore be replaced entirely by a
> %20 = add nsw i32 %9, 10000
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