[mips] Add support for accessing $gp as a named register.

Summary:
Mips Linux uses $gp to hold a pointer to thread info structure and accesses it
with a named register. This makes this work for LLVM.

The N32 ABI doesn't quite work yet since the frontend generates incorrect IR
for this case. It neglects to truncate the 64-bit GPR to a 32-bit value before
converting to a pointer. Given correct IR (as in the testcase in this patch),
it works correctly.

Reviewers: sstankovic, vmedic, atanasyan

Reviewed By: atanasyan

Subscribers: llvm-commits

Differential Revision: http://reviews.llvm.org/D6893

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@225529 91177308-0d34-0410-b5e6-96231b3b80d8

1 files changed, 17 insertions, 0 deletions

diff --git a/test/CodeGen/Mips/named-register-o32.ll b/test/CodeGen/Mips/named-register-o32.ll
new file mode 100644
index 00000000000..0890c66283c
--- /dev/null
+++ b/test/CodeGen/Mips/named-register-o32.ll
@@ -0,0 +1,17 @@
+; RUN: llc -march=mips -relocation-model=static -mattr=+noabicalls < %s | FileCheck %s
+
+define i32* @get_gp() {
+entry:
+  %0 = call i32 @llvm.read_register.i32(metadata !0)
+  %1 = inttoptr i32 %0 to i32*
+  ret i32* %1
+}
+
+; CHECK-LABEL: get_gp:
+; CHECK:           move $2, $gp
+
+declare i32 @llvm.read_register.i32(metadata)
+
+!llvm.named.register.$28 = !{!0}
+
+!0 = !{!"$28"}