Implement an address_insignificant attribute

This can be applied to statics and it will indicate that LLVM will attempt to
merge the constant in .data with other statics.

I have preliminarily applied this to all of the statics generated by the new
`ifmt!` syntax extension. I compiled a file with 1000 calls to `ifmt!` and a
separate file with 1000 calls to `fmt!` to compare the sizes, and the results
were:

fmt           310k
ifmt (before) 529k
ifmt (after)  202k

This now means that ifmt! is both faster and smaller than fmt!, yay!

Author: alexcrichton

src/librustc/lib/llvm.rs

@@ -2080,6 +2080,9 @@ pub mod llvm {
                                             Elements: ValueRef,
                                             RunTimeLang: c_uint)
                                             -> ValueRef;
+
+        #[fast_ffi]
+        pub fn LLVMSetUnnamedAddr(GlobalVar: ValueRef, UnnamedAddr: Bool);
     }
 }
 
@@ -2099,6 +2102,12 @@ pub fn SetLinkage(Global: ValueRef, Link: Linkage) {
     }
 }
 
+pub fn SetUnnamedAddr(Global: ValueRef, Unnamed: bool) {
+    unsafe {
+        llvm::LLVMSetUnnamedAddr(Global, Unnamed as Bool);
+    }
+}
+
 pub fn ConstICmp(Pred: IntPredicate, V1: ValueRef, V2: ValueRef) -> ValueRef {
     unsafe {
         llvm::LLVMConstICmp(Pred as c_ushort, V1, V2)

src/librustc/middle/trans/base.rs

@@ -2159,19 +2159,18 @@ pub fn trans_item(ccx: @mut CrateContext, item: &ast::item) {
       }
       ast::item_static(_, m, expr) => {
           consts::trans_const(ccx, m, item.id);
-          // Do static_assert checking. It can't really be done much earlier because we need to get
-          // the value of the bool out of LLVM
-          for attr in item.attrs.iter() {
-              if "static_assert" == attr.name() {
-                  if m == ast::m_mutbl {
-                      ccx.sess.span_fatal(expr.span,
-                                          "cannot have static_assert on a mutable static");
-                  }
-                  let v = ccx.const_values.get_copy(&item.id);
-                  unsafe {
-                      if !(llvm::LLVMConstIntGetZExtValue(v) as bool) {
-                          ccx.sess.span_fatal(expr.span, "static assertion failed");
-                      }
+          // Do static_assert checking. It can't really be done much earlier
+          // because we need to get the value of the bool out of LLVM
+          if attr::contains_name(item.attrs, "static_assert") {
+              if m == ast::m_mutbl {
+                  ccx.sess.span_fatal(expr.span,
+                                      "cannot have static_assert on a mutable \
+                                       static");
+              }
+              let v = ccx.const_values.get_copy(&item.id);
+              unsafe {
+                  if !(llvm::LLVMConstIntGetZExtValue(v) as bool) {
+                      ccx.sess.span_fatal(expr.span, "static assertion failed");
                   }
               }
           }
@@ -2432,6 +2431,15 @@ pub fn get_item_val(ccx: @mut CrateContext, id: ast::NodeId) -> ValueRef {
                                     llvm::LLVMAddGlobal(ccx.llmod, llty, buf)
                                 };
 
+                                // Apply the `unnamed_addr` attribute if
+                                // requested
+                                if attr::contains_name(i.attrs,
+                                                       "address_insignificant"){
+                                    lib::llvm::SetUnnamedAddr(g, true);
+                                    lib::llvm::SetLinkage(g,
+                                        lib::llvm::InternalLinkage);
+                                }
+
                                 ccx.item_symbols.insert(i.id, sym);
                                 g
                             }

src/libsyntax/ext/ifmt.rs

@@ -429,7 +429,12 @@ impl Context {
             let st = ast::item_static(ty, ast::m_imm, method);
             let static_name = self.ecx.ident_of(fmt!("__static_method_%u",
                                                      self.method_statics.len()));
-            let item = self.ecx.item(sp, static_name, ~[], st);
+            // Flag these statics as `address_insignificant` so LLVM can
+            // merge duplicate globals as much as possible (which we're
+            // generating a whole lot of).
+            let unnamed = self.ecx.meta_word(self.fmtsp, @"address_insignificant");
+            let unnamed = self.ecx.attribute(self.fmtsp, unnamed);
+            let item = self.ecx.item(sp, static_name, ~[unnamed], st);
             self.method_statics.push(item);
             self.ecx.expr_ident(sp, static_name)
         };
@@ -550,7 +555,10 @@ impl Context {
         let ty = self.ecx.ty(self.fmtsp, ty);
         let st = ast::item_static(ty, ast::m_imm, fmt);
         let static_name = self.ecx.ident_of("__static_fmtstr");
-        let item = self.ecx.item(self.fmtsp, static_name, ~[], st);
+        // see above comment for `address_insignificant` and why we do it
+        let unnamed = self.ecx.meta_word(self.fmtsp, @"address_insignificant");
+        let unnamed = self.ecx.attribute(self.fmtsp, unnamed);
+        let item = self.ecx.item(self.fmtsp, static_name, ~[unnamed], st);
         let decl = respan(self.fmtsp, ast::decl_item(item));
         lets.push(@respan(self.fmtsp, ast::stmt_decl(@decl, self.ecx.next_id())));
 
@@ -613,6 +621,7 @@ impl Context {
         if ty == Unknown {
             ty = Known(@"?");
         }
+
         let argptr = self.ecx.expr_addr_of(sp, self.ecx.expr_ident(sp, ident));
         match ty {
             Known(tyname) => {
@@ -685,7 +694,7 @@ pub fn expand_syntax_ext(ecx: @ExtCtxt, sp: span,
     };
     cx.fmtsp = efmt.span;
     let fmt = expr_to_str(ecx, efmt,
-                          ~"first argument to ifmt! must be a string literal.");
+                          "first argument to ifmt! must be a string literal.");
 
     let mut err = false;
     do parse::parse_error::cond.trap(|m| {

src/rustllvm/RustWrapper.cpp

@@ -833,3 +833,7 @@ extern "C" LLVMValueRef LLVMDIBuilderCreateUnionType(
         unwrapDI<DIArray>(Elements),
         RunTimeLang));
 }
+
+extern "C" void LLVMSetUnnamedAddr(LLVMValueRef Value, LLVMBool Unnamed) {
+    unwrap<GlobalValue>(Value)->setUnnamedAddr(Unnamed);
+}

src/rustllvm/rustllvm.def.in

@@ -613,3 +613,4 @@ LLVMDIBuilderInsertDeclareBefore
 LLVMDIBuilderCreateEnumerator
 LLVMDIBuilderCreateEnumerationType
 LLVMDIBuilderCreateUnionType
+LLVMSetUnnamedAddr