Allow sret on the second parameter as well as the first

MSVC always places the implicit sret parameter after the implicit this
parameter of instance methods.  We used to handle this for
x86_thiscallcc by allocating the sret parameter on the stack and leaving
the this pointer in ecx, but that doesn't handle alternative calling
conventions like cdecl, stdcall, fastcall, or the win64 convention.

Instead, change the verifier to allow sret on the second parameter.

This also requires changing the Mips and X86 backends to return the
argument with the sret parameter, instead of assuming that the sret
parameter comes first.

The Sparc backend also returns sret parameters in a register, but I
wasn't able to update it to handle secondary sret parameters.  It
currently calls report_fatal_error if you feed it an sret in the second
parameter.

Reviewers: rafael.espindola, majnemer

Differential Revision: http://reviews.llvm.org/D3617

llvm-svn: 208453

2 files changed, 11 insertions, 6 deletions

diff --git a/llvm/lib/IR/Function.cpp b/llvm/lib/IR/Function.cpp
index da7774128a1..c1a09686b02 100644
--- a/llvm/lib/IR/Function.cpp
+++ b/llvm/lib/IR/Function.cpp
@@ -207,10 +207,10 @@ void Function::eraseFromParent() {
 // Function Implementation
 //===----------------------------------------------------------------------===//
 
-Function::Function(FunctionType *Ty, LinkageTypes Linkage, const Twine &name,
-                   Module *ParentModule)
-    : GlobalValue(PointerType::getUnqual(Ty), Value::FunctionVal, nullptr, 0,
-                  Linkage, name) {
+Function::Function(FunctionType *Ty, LinkageTypes Linkage,
+                   const Twine &name, Module *ParentModule)
+  : GlobalValue(PointerType::getUnqual(Ty),
+                Value::FunctionVal, nullptr, 0, Linkage, name) {
   assert(FunctionType::isValidReturnType(getReturnType()) &&
          "invalid return type");
   SymTab = new ValueSymbolTable();
diff --git a/llvm/lib/IR/Verifier.cpp b/llvm/lib/IR/Verifier.cpp
index 083f7b5255e..ec287f58fa3 100644
--- a/llvm/lib/IR/Verifier.cpp
+++ b/llvm/lib/IR/Verifier.cpp
@@ -820,6 +820,7 @@ void Verifier::VerifyFunctionAttrs(FunctionType *FT, AttributeSet Attrs,
 
   bool SawNest = false;
   bool SawReturned = false;
+  bool SawSRet = false;
 
   for (unsigned i = 0, e = Attrs.getNumSlots(); i != e; ++i) {
     unsigned Idx = Attrs.getSlotIndex(i);
@@ -850,8 +851,12 @@ void Verifier::VerifyFunctionAttrs(FunctionType *FT, AttributeSet Attrs,
       SawReturned = true;
     }
 
-    if (Attrs.hasAttribute(Idx, Attribute::StructRet))
-      Assert1(Idx == 1, "Attribute sret is not on first parameter!", V);
+    if (Attrs.hasAttribute(Idx, Attribute::StructRet)) {
+      Assert1(!SawSRet, "Cannot have multiple 'sret' parameters!", V);
+      Assert1(Idx == 1 || Idx == 2,
+              "Attribute 'sret' is not on first or second parameter!", V);
+      SawSRet = true;
+    }
 
     if (Attrs.hasAttribute(Idx, Attribute::InAlloca)) {
       Assert1(Idx == FT->getNumParams(),