[ThinLTO] Stop importing constant global vars as copies in the backend
Summary:
We were doing an optimization in the ThinLTO backends of importing
constant unnamed_addr globals unconditionally as a local copy (regardless
of whether the thin link decided to import them). This should be done in
the thin link instead, so that resulting exported references are marked
and promoted appropriately, but will need a summary enhancement to mark
these variables as constant unnamed_addr.
The function import logic during the thin link was trying to handle
this proactively, by conservatively marking all values referenced in
the initializer lists of exported global variables as also exported.
However, this only handled values referenced directly from the
initializer list of an exported global variable. If the value is itself
a constant unnamed_addr variable, we could end up exporting its
references as well. This caused multiple issues. The first is that the
transitively exported references weren't promoted. Secondly, some could
not be promoted/renamed (e.g. they had a section or other constraint).
recursively, instead of just adding the first level of initializer list
references to the ExportList directly.
Remove this optimization and the associated handling in the function
import backend. SPEC measurements indicate we weren't getting much
from it in any case.
Fixes PR31052.
Reviewers: mehdi_amini
Subscribers: krasin, llvm-commits
Differential Revision: https://reviews.llvm.org/D26880
llvm-svn: 288446
diff --git a/llvm/test/Linker/funcimport.ll b/llvm/test/Linker/funcimport.ll
index 53dbc6d..6d9395f 100644
--- a/llvm/test/Linker/funcimport.ll
+++ b/llvm/test/Linker/funcimport.ll
@@ -14,7 +14,8 @@
; constant variable need promotion).
; RUN: llvm-link %t.bc -summary-index=%t3.thinlto.bc -S | FileCheck %s --check-prefix=EXPORTSTATIC
; EXPORTSTATIC-DAG: @staticvar.llvm.{{.*}} = hidden global
-; EXPORTSTATIC-DAG: @staticconstvar = internal unnamed_addr constant
+; Eventually @staticconstvar can be exported as a copy and not promoted
+; EXPORTSTATIC-DAG: @staticconstvar.llvm.0 = hidden unnamed_addr constant
; EXPORTSTATIC-DAG: @P.llvm.{{.*}} = hidden global void ()* null
; EXPORTSTATIC-DAG: define hidden i32 @staticfunc.llvm.
; EXPORTSTATIC-DAG: define hidden void @staticfunc2.llvm.
@@ -68,7 +69,8 @@
; promoted and renamed (including static constant variable).
; RUN: llvm-link %t2.bc -summary-index=%t3.thinlto.bc -import=referencestatics:%t.bc -S | FileCheck %s --check-prefix=IMPORTSTATIC
; IMPORTSTATIC-DAG: @staticvar.llvm.{{.*}} = external hidden global
-; IMPORTSTATIC-DAG: @staticconstvar.llvm.{{.*}} = internal unnamed_addr constant
+; Eventually @staticconstvar can be imported as a copy
+; IMPORTSTATIC-DAG: @staticconstvar.llvm.{{.*}} = external hidden unnamed_addr constant
; IMPORTSTATIC-DAG: define available_externally i32 @referencestatics
; IMPORTSTATIC-DAG: %call = call i32 @staticfunc.llvm.
; IMPORTSTATIC-DAG: %0 = load i32, i32* @staticvar.llvm.