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DeadArgElim: aggregate Return assessment properly. 

I mistakenly thought the liveness of each "RetVal(F, i)" depended only on F. It
actually depends on the index too, which means we need to be careful about how
the results are combined before return. In particular if a single Use returns
Live, that counts for the entire object, at the granularity we're considering.

llvm-svn: 228885
This commit is contained in:
Tim Northover 2015-02-11 23:13:11 +00:00
parent ef6baea74e
commit 02438033e8
2 changed files with 37 additions and 4 deletions
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11
llvm/lib/Transforms/IPO/DeadArgumentElimination.cpp
View File

@ -449,12 +449,15 @@ DAE::Liveness DAE::SurveyUse(const Use *U,
// We might be live, depending on the liveness of Use.
return MarkIfNotLive(Use, MaybeLiveUses);
} else {
DAE::Liveness Result;
DAE::Liveness Result = MaybeLive;
for (unsigned i = 0; i < NumRetVals(F); ++i) {
RetOrArg Use = CreateRet(F, i);
// We might be live, depending on the liveness of Use. All Results
// should be the same since they depend only on F.
Result = MarkIfNotLive(Use, MaybeLiveUses);
// We might be live, depending on the liveness of Use. If any
// sub-value is live, then the entire value is considered live. This
// is a conservative choice, and better tracking is possible.
DAE::Liveness SubResult = MarkIfNotLive(Use, MaybeLiveUses);
if (Result != Live)
Result = SubResult;
}
return Result;
}

30
llvm/test/Transforms/DeadArgElim/aggregates.ll
View File

@ -130,3 +130,33 @@ define i32 @test_ret_applies_to_all() {
%ret = extractvalue {i32, i32} %val, 1
ret i32 %ret
}
; Case 6: When considering @mid, the return instruciton has sub-value 0
; unconditionally live, but 1 only conditionally live. Since at that level we're
; applying the results to the whole of %res, this means %res is live and cannot
; be reduced. There is scope for further optimisation here (though not visible
; in this test-case).
; CHECK-LABEL: define internal { i8*, i32 } @inner()
define internal {i8*, i32} @mid() {
%res = call {i8*, i32} @inner()
%intval = extractvalue {i8*, i32} %res, 1
%tst = icmp eq i32 %intval, 42
br i1 %tst, label %true, label %true
true:
ret {i8*, i32} %res
}
define internal {i8*, i32} @inner() {
ret {i8*, i32} {i8* null, i32 42}
}
define internal i8 @outer() {
%res = call {i8*, i32} @mid()
%resptr = extractvalue {i8*, i32} %res, 0
%val = load i8* %resptr
ret i8 %val
}