Make global aliases have symbol size equal to their type

[john.brawn]

Summary

This is mainly for the benefit of GlobalMerge, so that an alias into a MergedGlobals variable has the same size as the original non-merged variable.

[• rafael]

Why do you need to print the .size in the assembly? The assembler
itself (MC/gas) propagates the size.

[john.brawn]

In D10837#197109, @rafael wrote:

Why do you need to print the .size in the assembly? The assembler
itself (MC/gas) propagates the size.

Yes, but it propagates it from the aliasee. In global-merge-2.ll, for example, if you generate object output then inspect the symbol table it says

0000000000000000 g .bss 0000000c x
0000000000000004 g .bss 0000000c y
0000000000000008 g .bss 0000000c z

which says that x is 12-bytes in size starting at offset 8 inside the .bss section. As x was originally (before GlobalMerge) a 4-byte variable this is slightly surprising, plus the .bss section is 12 bytes long so z apparently extends past the end of the .bss section.

[john.brawn]

which says that x is 12-bytes in size starting at offset 8 inside the .bss section. As x was originally (before GlobalMerge) a 4-byte variable this is slightly surprising, plus the .bss section is 12 bytes long so z apparently extends past the end of the .bss section.

That should have been 'z is 12-bytes in size starting at offset 8'.

[• rafael]

I see,

Given

.size b, 8

b:

.quad 4
a = b + 4

The assembler says that "a" has size 8 :-(

I already think that the assembler is trying to do too much in here,
so I agree that printing a .size is the way to go.

const DataLayout *DL = TM.getDataLayout();

Please get the DataLayout from the Module.

David, would this be in your way for removing types from pointers? If not, LGTM.

Thanks,
Rafael

[john.brawn]

Adjusted to get the DataLayout from the Module.

[john.brawn]

Ping.

[dblaikie]

lib/CodeGen/AsmPrinter/AsmPrinter.cpp

1117

Yeah, this would be a problem for the typeless pointer work. If you can avoid introducing calls to PointerType::getElementType it'll make my migration work a bit easier.

Chances are, what we need, is GlobalValue to have a value type (I think I've added this to GlobalVariable or some other things in the GlobalValue hierarchy - if all of them need it, we can just add it up in GlobalValue) that can be retrieved directly, because their actual type will eventually just be an opaque pointer.

I don't know much about aliases - they can be differently typed than the thing they alias? Not sure what that means or what their size means either...

[• rafael]

I don't know much about aliases - they can be differently typed than the thing they alias?

Yes.

Not sure what that means or what their size means either...

Pretty much whatever the "user" wants it to mean. In this case the
only real user we have is the concatenation of global variables, so
the desire is for the size to represent the original size.

Cheers,
Rafael

[john.brawn]

lib/CodeGen/AsmPrinter/AsmPrinter.cpp

1117

It looks like you've added getValueType() to GlobalValue, which currently just returns getType()->getElementType() though I guess that will be changed sometime in the future. I'll use that instead.

[john.brawn]

Use getValueType() instead getType()->getElementType().

[• rafael]

LGTM

lib/CodeGen/AsmPrinter/AsmPrinter.cpp

else if (Alias.hasWeakLinkage() || Alias.hasLinkOnceLinkage())
       OutStreamer->EmitSymbolAttribute(Name, MCSA_WeakReference);
     else
       assert(Alias.hasLocalLinkage() && "Invalid alias linkage");
 
     EmitVisibility(Name, Alias.getVisibility());
 
     // Emit the directives as assignments aka .set:
     OutStreamer->EmitAssignment(Name, lowerConstant(Alias.getAliasee()));
+
+    // Set the size of the alias symbol if we can, as otherwise the alias gets
+    // the size of the aliasee which may not be correct e.g. if the alias is of
+    // a member of a struct.
+    if (MAI->hasDotTypeDotSizeDirective() &&
+        Alias.getType()->getElementType()->isSized()) {
+      const DataLayout &DL = M.getDataLayout();
+      uint64_t Size = DL.getTypeAllocSize(Alias.getType()->getElementType());
+      OutStreamer->emitELFSize(cast<MCSymbolELF>(Name),
+                               MCConstantExpr::create(Size, OutContext));
+    }
   }
 
   GCModuleInfo *MI = getAnalysisIfAvailable<GCModuleInfo>();
   assert(MI && "AsmPrinter didn't require GCModuleInfo?");
   for (GCModuleInfo::iterator I = MI->end(), E = MI->begin(); I != E; )
     if (GCMetadataPrinter *MP = GetOrCreateGCPrinter(**--I))
       MP->finishAssembly(M, *MI, *this);
 

test/CodeGen/AArch64/global-merge-2.ll

 ;CHECK: .type _MergedGlobals_x,@object // @_MergedGlobals_x
 ;CHECK: .globl  _MergedGlobals_x
 ;CHECK: .align  3
 ;CHECK: _MergedGlobals_x:
 ;CHECK: .size _MergedGlobals_x, 12
 
 ;CHECK: .globl  x
 ;CHECK: x = _MergedGlobals_x
+;CHECK: .size x, 4
 ;CHECK: .globl  y
 ;CHECK: y = _MergedGlobals_x+4
+;CHECK: .size y, 4
 ;CHECK: .globl  z
 ;CHECK: z = _MergedGlobals_x+8
+;CHECK: .size z, 4
 
 ;CHECK-APPLE-IOS: .globl  __MergedGlobals_x       ; @_MergedGlobals_x
 ;CHECK-APPLE-IOS: .zerofill __DATA,__common,__MergedGlobals_x,12,3
 
 ;CHECK-APPLE-IOS: .globl  _x
 ;CHECK-APPLE-IOS: _x = __MergedGlobals_x
 ;CHECK-APPLE-IOS: .globl  _y
 ;CHECK-APPLE-IOS: _y = __MergedGlobals_x+4
 ;CHECK-APPLE-IOS: .globl  _z
 ;CHECK-APPLE-IOS: _z = __MergedGlobals_x+8
 ;CHECK-APPLE-IOS: .subsections_via_symbols

test/CodeGen/AArch64/global-merge-3.ll

 ;CHECK-APPLE-IOS: .long 1
 ;CHECK-APPLE-IOS: .space  4000
 
 ;CHECK-APPLE-IOS: .globl  __MergedGlobals_y       ; @_MergedGlobals_y
 ;CHECK-APPLE-IOS: .zerofill __DATA,__common,__MergedGlobals_y,4000,4
 
 ;CHECK: .globl  x
 ;CHECK: x = _MergedGlobals_x+4
+;CHECK: .size x, 4000
 ;CHECK: .globl  y
 ;CHECK: y = _MergedGlobals_y
+;CHECK: .size y, 4000
 
 ;CHECK-APPLE-IOS:.globl _x
 ;CHECK-APPLE-IOS: _x = __MergedGlobals_x+4
 ;CHECK-APPLE-IOS:.globl _y
 ;CHECK-APPLE-IOS: _y = __MergedGlobals_y

test/CodeGen/ARM/aliases.ll

 ; RUN: llc < %s -mtriple=arm-linux-gnueabi | FileCheck %s
 
 ; CHECK: .globl test
 
+; CHECK: .globl structvar
+; CHECK: .size structvar, 8
+
 ; CHECK: .globl foo1
 ; CHECK: foo1 = bar
+; CHECK: .size foo1, 4
 
 ; CHECK: .globl foo2
 ; CHECK: foo2 = bar
+; CHECK: .size foo2, 4
 
 ; CHECK: .weak  bar_f
 ; CHECK: bar_f = foo_f
 
 ; CHECK: bar_i = bar
+; CHECK: .size bar_i, 4
 
 ; CHECK: .globl A
 ; CHECK: A = bar
+; CHECK: .size A, 8
+
+; CHECK: .globl elem0
+; CHECK: elem0 = structvar
+; CHECK: .size elem0, 4
+
+; CHECK: .globl elem1
+; CHECK: elem1 = structvar+4
+; CHECK: .size elem1, 4
 
 @bar = global i32 42
 @foo1 = alias i32* @bar
 @foo2 = alias i32* @bar
 
 %FunTy = type i32()
 
 define i32 @foo_f() {
   ret i32 0
 }
 @bar_f = weak alias %FunTy* @foo_f
 
 @bar_i = internal alias i32* @bar
 
 @A = alias bitcast (i32* @bar to i64*)
 
+@structvar = global {i32, i32} {i32 1, i32 2}
+@elem0 = alias getelementptr({i32, i32}, {i32, i32}*  @structvar, i32 0, i32 0)
+@elem1 = alias getelementptr({i32, i32}, {i32, i32}*  @structvar, i32 0, i32 1)
+
 define i32 @test() {
 entry:
    %tmp = load i32, i32* @foo1
    %tmp1 = load i32, i32* @foo2
    %tmp0 = load i32, i32* @bar_i
    %tmp2 = call i32 @foo_f()
    %tmp3 = add i32 %tmp, %tmp2
    %tmp4 = call i32 @bar_f()
    %tmp5 = add i32 %tmp3, %tmp4
    %tmp6 = add i32 %tmp1, %tmp5
    %tmp7 = add i32 %tmp6, %tmp0
    ret i32 %tmp7
 }