[NVPTX] Use .common linkage for common globals (#84416)

Switch from `.weak` to `.common` linkage for common global variables
where possible. The `.common` linkage is described in [PTX ISA 11.6.4.
Linking Directives:
.common](https://docs.nvidia.com/cuda/parallel-thread-execution/index.html#linking-directives-common)
> Declares identifier to be globally visible but “common”.
>
>Common symbols are similar to globally visible symbols. However
multiple object files may declare the same common symbol and they may
have different types and sizes and references to a symbol get resolved
against a common symbol with the largest size.
>
>Only one object file can initialize a common symbol and that must have
the largest size among all other definitions of that common symbol from
different object files.
>
>.common linking directive can be used only on variables with .global
storage. It cannot be used on function symbols or on symbols with opaque
type.

Author: AlexMaclean

llvm/lib/Target/NVPTX/NVPTXAsmPrinter.cpp

@@ -1019,14 +1019,16 @@ void NVPTXAsmPrinter::printModuleLevelGV(const GlobalVariable *GVar,
   const DataLayout &DL = getDataLayout();
 
   // GlobalVariables are always constant pointers themselves.
-  PointerType *PTy = GVar->getType();
   Type *ETy = GVar->getValueType();
 
   if (GVar->hasExternalLinkage()) {
     if (GVar->hasInitializer())
       O << ".visible ";
     else
       O << ".extern ";
+  } else if (GVar->hasCommonLinkage() &&
+             GVar->getAddressSpace() == ADDRESS_SPACE_GLOBAL) {
+    O << ".common ";
   } else if (GVar->hasLinkOnceLinkage() || GVar->hasWeakLinkage() ||
              GVar->hasAvailableExternallyLinkage() ||
              GVar->hasCommonLinkage()) {
@@ -1138,7 +1140,7 @@ void NVPTXAsmPrinter::printModuleLevelGV(const GlobalVariable *GVar,
   }
 
   O << ".";
-  emitPTXAddressSpace(PTy->getAddressSpace(), O);
+  emitPTXAddressSpace(GVar->getAddressSpace(), O);
 
   if (isManaged(*GVar)) {
     if (STI.getPTXVersion() < 40 || STI.getSmVersion() < 30) {
@@ -1167,8 +1169,8 @@ void NVPTXAsmPrinter::printModuleLevelGV(const GlobalVariable *GVar,
     // Ptx allows variable initilization only for constant and global state
     // spaces.
     if (GVar->hasInitializer()) {
-      if ((PTy->getAddressSpace() == ADDRESS_SPACE_GLOBAL) ||
-          (PTy->getAddressSpace() == ADDRESS_SPACE_CONST)) {
+      if ((GVar->getAddressSpace() == ADDRESS_SPACE_GLOBAL) ||
+          (GVar->getAddressSpace() == ADDRESS_SPACE_CONST)) {
         const Constant *Initializer = GVar->getInitializer();
         // 'undef' is treated as there is no value specified.
         if (!Initializer->isNullValue() && !isa<UndefValue>(Initializer)) {
@@ -1183,7 +1185,7 @@ void NVPTXAsmPrinter::printModuleLevelGV(const GlobalVariable *GVar,
             !isa<UndefValue>(GVar->getInitializer())) {
           report_fatal_error("initial value of '" + GVar->getName() +
                              "' is not allowed in addrspace(" +
-                             Twine(PTy->getAddressSpace()) + ")");
+                             Twine(GVar->getAddressSpace()) + ")");
         }
       }
     }
@@ -1202,8 +1204,8 @@ void NVPTXAsmPrinter::printModuleLevelGV(const GlobalVariable *GVar,
       ElementSize = DL.getTypeStoreSize(ETy);
       // Ptx allows variable initilization only for constant and
       // global state spaces.
-      if (((PTy->getAddressSpace() == ADDRESS_SPACE_GLOBAL) ||
-           (PTy->getAddressSpace() == ADDRESS_SPACE_CONST)) &&
+      if (((GVar->getAddressSpace() == ADDRESS_SPACE_GLOBAL) ||
+           (GVar->getAddressSpace() == ADDRESS_SPACE_CONST)) &&
           GVar->hasInitializer()) {
         const Constant *Initializer = GVar->getInitializer();
         if (!isa<UndefValue>(Initializer) && !Initializer->isNullValue()) {

llvm/test/CodeGen/NVPTX/common-linkage.ll

@@ -0,0 +1,26 @@
+; RUN: llc < %s -march=nvptx -mcpu=sm_20 | FileCheck %s
+; RUN: %if ptxas %{ llc < %s -march=nvptx64 -mcpu=sm_20 | %ptxas-verify %}
+
+; CHECK: .common .global .align 4 .u32 g
+@g = common addrspace(1) global i32 0, align 4
+
+; CHECK: .weak .const .align 4 .u32 c
+@c = common addrspace(4) global i32 0, align 4
+
+; CHECK: .weak .shared .align 4 .u32 s
+@s = common addrspace(3) global i32 0, align 4
+
+define i32 @f1() {
+  %1 = load i32, ptr addrspace(1) @g
+  ret i32 %1
+}
+
+define i32 @f4() {
+  %1 = load i32, ptr addrspace(4) @c
+  ret i32 %1
+}
+
+define i32 @f3() {
+  %1 = load i32, ptr addrspace(3) @s
+  ret i32 %1
+}

llvm/test/CodeGen/NVPTX/weak-global.ll

@@ -1,7 +1,7 @@
 ; RUN: llc < %s -march=nvptx64 -mcpu=sm_20 | FileCheck %s
 ; RUN: %if ptxas %{ llc < %s -march=nvptx64 -mcpu=sm_20 | %ptxas-verify %}
 
-; CHECK: .weak .global .align 4 .u32 g
+; CHECK: .common .global .align 4 .u32 g
 @g = common addrspace(1) global i32 zeroinitializer
 
 define i32 @func0() {