Hi Shi,
Your fix by calling CODEMAP rename is OK in its own right. But this fix
is in reality for patching up a bug that was introduced by copy
propagation. We should fix the root cause of the problem, NOT to patch
it so the symptom does not appear.
Basically, the following from your original mail:
STMT1
sym2v4 = chi(sym2v3) NOT LIVE
default_vsym_v4 = chi(default_vsym_v3) LIVE
is already wrong. sym2v aliases with the default vsym. Thus, one
cannot be live while the other is dead.
In looking at the code in opt_dse.cxx, I found that it has diverged a
lot from the version that I'm familiar with. Thus, I'm not in a
position to point out what the root problem is. At least the version I
worked with at PathScale does not have this problem. I would welcome
others to provide input on this.
Fred
On 07/04/2011 07:37 PM, Hui Shi wrote:
Hi Fred,
Thanks for your comments.
The reanming happens only when there are some dead phi/chi nodes
revive in CODEMAP::Iload_folded.
To evaluate how this patch will affect compile-time, I add dump when
this renaming happens.
Tested with SPEC2006 base/peak
Following cases have CODEMAP rename after create CODEMAPE, and
renaming count is seldom consider test's PU count..
447.dealII base/peak rename 20+ times
483.xalancbmk base/peak rename 30+ times
416.gamess peak rename 4 times
458.sjeng base rename 1 time.
So this renaming rarely happens and I think we can afford the
increased compile time.
And from another perspective. Rename_CODEMAP is already used in other
phase like aft CGF optimization, COMP_UNIT::Fold_lda_iload_istore, etc.
These optimizations and renaming are more likely happen.
One utiIity I hope WOPT can have is CODEMAP rename only rename somve
AUX (give in a table or list). Then rename cost can be decreased a lot.
For copy propagation issue.
The copy propagation happens in Dead_Store_Elminination,
DSE::Propagate_vsym_bb.
To disable the propagation, need change Dead_Store_Elimination. If one
chi node is live,
all other chi nodes binding to same statment and set_required should
live because they are aliased with each other.
This will also cause compile time isuee, because more live phi/chi
node in codrep.
Another issue is though we fix the copy propgation problem, the revive
logical is still there in CODEMAP::Iload_folded.
There still possible dead phi/chi nodes revive in CODEMAP creation. So
complete fix is rename after create CODEMAP.
The main issue is create_codemap not simply create coderep, it also
perform iload folding may change version.
If iload folding function is totolay peformed in
COMP_UNIT::Fold_lda_iload_istore, it has no problem.
Regards
Shi Hui
On Mon, Jul 4, 2011 at 9:26 PM, Fred Chow <frdc...@gmail.com
<mailto:frdc...@gmail.com>> wrote:
Doing renaming is quite compile-time expensive, because it is like
running the SSA construction algorithm. The problem your
mentioned would not have occurred if sym2v3 has not been
propagated down to STMT3. I don't recall we were doing copy
propagation for chi operands. The overlapped live range was
caused by this bad copy propagation, NOT by the ILOAD folding.
The above copy propagation is bad because sym2v4 is NOT dead,
because it aliases with the default vsym which appears in the mu
of STMT2. So your fix should be in preventing the copy propagation.
Fred
On 06/29/2011 11:35 PM, Hui Shi wrote:
Could gatekeeper help review this fix?
In SSA::Create_CODEMAP ILOAD folding is performed during coderep
setup.
The problem is iload folding may revive some dead phi/chi node.
For example
STMT1
sym2v4 = chi(sym2v3) NOT LIVE
default_vsym_v4 = chi(default_vsym_v3) LIVE
....
STMT2
ILOAD mu(default_vsym_v4)
...
STMT3
sym2v5 = chi(sym2v3) LIVE // sym2v3 is propagate down from above
dead chi.
default_vsym_v5 = chi(default_vsym_v4)
If sym2v4 may use at STMT2, ILOAD's MU node must on STMT1's chi list
(because MU node must alias with sym2, otherwise its alias issue).
And MU node's corresponding CHI node must live on STMT1, because
set_required_mu will set chi node live.
So it's okay for DSE to mark fisrt CHI dead if STMT1 and STMT3
write same memory location.
1. It will not make STMT1 dead, because the other chi node is live.
2. ILOAD's MU node's def statment still STMT1
If ILOAD folding transform ILOAD to LDID and LDID's AUX is sym2.
ILOD_FOLDING revives dead chi sym2v4 = chi(sym2v3) without update
the late chi node's opnd.
This introduce overlapped between sym2v3 and sym2v4.
Fix is rename CODEMAP if iload folding revive dead phi/chi node.
OPT_REVISE_SSA::Fold_lda_indirects also rename CODEMAP after
optimization.
Fix is
1. record in htable, if dead chi/phi node is revived in
SSA::Create_CODEMAP.
This only happends when ILOAD_Folding happens and find
corresponding chi/phi is dead.
2. After SSA::Create_CODEMAP, rename coderep if dead chi/phi node
is revive in CODEMAP setup.
Different with fixing in DSE to mark first chi node live, this
fix not affect DSE, so not affect performance.
And this is verified with spec2006.rate on xeon.
Regards
Shi Hui
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