llvm dev - Jul 2011 - [LLVMdev] Missed optimization with indirectbr terminator

Consider this IR fragment produced after -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).
Is this a known missed optimization?

-- 
Carlo Alberto Ferraris <cafxx at strayorange.com 
<mailto:cafxx at strayorange.com>>
website/blog <http://cafxx.strayorange.com> - +39 333 7643 235
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On Jul 7, 2011, at 2:05 AM, Carlo Alberto Ferraris wrote:
> Consider this IR fragment produced after -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).
> Is this a known missed optimization?
I haven't read the code in detail, but it looks like JumpThreading at least
attempts to thread across indirect branches. You can either try to fix it or
file a bug with your test case.

Cameron
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Il 07/07/2011 11:14, Cameron Zwarich ha scritto:
> I haven't read the code in detail, but it looks like JumpThreading at 
> least attempts to thread across indirect branches. You can either try 
> to fix it or file a bug with your test case.
In the source it says "If the predecessor is an indirect goto, we can't
split the edge. 
<http://llvm.org/docs/doxygen/html/JumpThreading_8cpp_source.html#l00914>"
Why would that be the case?
-- 
Carlo Alberto Ferraris <cafxx at strayorange.com 
<mailto:cafxx at strayorange.com>>
website/blog <http://cafxx.strayorange.com> - +39 333 7643 235
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Il 07/07/2011 11:05, Carlo Alberto Ferraris ha scritto:
> Is this a known missed optimization?
Filed bug 10377 <http://llvm.org/bugs/show_bug.cgi?id=10377> w/testcase.

-- 
Carlo Alberto Ferraris <cafxx at strayorange.com 
<mailto:cafxx at strayorange.com>>
website/blog <http://cafxx.strayorange.com> - +39 333 7643 235
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