[mips] Handle variables with an explicit section and interactions with .sdata, .sbss

[sdardis]

Summary

If a variable has an explicit section such as .sdata or .sbss, it is placed
in that section and accessed in a gp relative manner. This overrides the global
-G setting.

Otherwise if a variable has a explicit section attached to it, such as '.rodata'
or '.mysection', it is not placed in the small data section. This also overrides
the global -G setting.

[atanasyan]

LGTM

llvm/trunk/lib/Target/Mips/MipsTargetObjectFile.cpp

IsGlobalInSmallSectionImpl(const GlobalObject *GO,
   if (!Subtarget.useSmallSection())
     return false;
 
   // Only global variables, not functions.
   const GlobalVariable *GVA = dyn_cast<GlobalVariable>(GO);
   if (!GVA)
     return false;
 
+  // If the variable has an explicit section, it is placed in that section but
+  // it's addressing mode may change.
+  if (GVA->hasSection()) {
+    StringRef Section = GVA->getSection();
+
+    // Explicitly placing any variable in the small data section overrides
+    // the global -G value.
+    if (Section == ".sdata" || Section == ".sbss")
+      return true;
+
+    // Otherwise reject accessing it through the gp pointer. There are some
+    // historic cases which GCC doesn't appear to respect any more. These
+    // are .lit4, .lit8 and .srdata. For the moment reject these as well.
+    return false;
+  }
+
   // Enforce -mlocal-sdata.
   if (!LocalSData && GVA->hasLocalLinkage())
     return false;
 
   // Enforce -mextern-sdata.
   if (!ExternSData && ((GVA->hasExternalLinkage() && GVA->isDeclaration()) ||
                        GVA->hasCommonLinkage()))
     return false;

llvm/trunk/test/CodeGen/Mips/gpopt-explict-section.ll

+; RUN: llc < %s -march=mips -mcpu=mips32 -mips-ssection-threshold=8 \
+; RUN:     -relocation-model=static -mattr=+noabicalls -mgpopt \
+; RUN:   | FileCheck %s
+
+; Test that object with an explicit section that is not .sdata or .sbss are not
+; considered in the small data section if they would otherwise qualify to be in
+; small data section. Also test that explicitly placing something in the small
+; data section uses %gp_rel addressing mode.
+
+@a = global [2 x i32] zeroinitializer, section ".rodata", align 4
+@b = global [4 x i32] zeroinitializer, section ".sdata", align 4
+@c = global [4 x i32] zeroinitializer, section ".sbss", align 4
+
+; CHECK-LABEL: g
+; CHECK:       lui $[[R:[0-9]+]], %hi(a)
+; CHECK:       lw  ${{[0-9]+}}, %lo(a)($[[R]])
+
+define i32 @g() {
+entry:
+  %0 = load i32, i32* getelementptr inbounds ([2 x i32], [2 x i32]* @a, i32 0, i32 0), align 4
+  ret i32 %0
+}
+
+; CHECK-LABEL: f:
+; CHECK-LABEL: lw ${{[0-9]+}}, %gp_rel(b)($gp)
+
+define i32 @f() {
+entry:
+  %0 = load i32, i32* getelementptr inbounds ([4 x i32], [4 x i32]* @b, i32 0, i32 0), align 4
+  ret i32 %0
+}
+
+; CHECK-LABEL: h:
+; CHECK-LABEL: lw ${{[0-9]+}}, %gp_rel(c)($gp)
+
+define i32 @h() {
+entry:
+  %0 = load i32, i32* getelementptr inbounds ([4 x i32], [4 x i32]* @c, i32 0, i32 0), align 4
+  ret i32 %0
+}
+
+
+; CHECK:  .type a,@object
+; CHECK:  .section  .rodata,"a",@progbits
+; CHECK:  .globl  a
+
+; CHECK:  .type b,@object
+; CHECK:  .section  .sdata,"aw",@progbits
+; CHECK:  .globl  b
+
+; CHECK:  .type c,@object
+; CHECK:  .section  .sbss,"aw",@nobits
+; CHECK:  .globl  c