[X86] Use RIP-relative addressing for data under large data threshold for medium code model

[aeubanks]

Summary

Since those data are assumed to be within the relocation offset limit.

[aeubanks]

depends on https://reviews.llvm.org/D149288

[jyknight]

llvm/lib/Target/X86/X86ISelLowering.cpp

20828

This function is getting kinda complex. And given we already have classifyLocalReference/classifyGlobalReference and friends, why does this function even need to look at GV and the CodeModel at all? That kinda feels wrong...Shouldn't it be entirely based on the OpFlags?

That is, I'm kinda wondering why this function isn't, in its entirety just

if (Subtarget.isPICStyleRIPRel() && OpFlags == X86II::MO_NO_FLAG)
  return X86ISD::WrapperRIP;

if (OpFlags == X86II::MO_GOTPCREL || OpFlags == X86II::MO_GOTPCREL_NORELAX)
  return X86ISD::WrapperRIP;

return X86ISD::Wrapper;

Or do we actually emit references that are neither GOT nor pc-relative in PIC mode in some circumstances? (Which would, then, imply text relocations...which seems wrong...)

[aeubanks]

clean up X86TargetLowering::getGlobalWrapperKind

[aeubanks]

rebase

[rnk]

llvm/lib/Target/X86/X86ISelLowering.cpp

20826–20828

I guess James suggested that we avoid looking at GV, which is how we knew we could do the rip-rel LEA previously.

llvm/test/CodeGen/X86/code-model-elf.ll

303

I think the old sequence is better, if we can get it. GCC does something weird:
https://gcc.godbolt.org/z/xPT49jqrT

I think they assume the program is in the low 4GB of the address space if compiled without PIC.

346

This seems undesirable. We should know that because global_fn is code, it is within 2GB of the current PC, so we can use the RIP, which has a smaller encoding (32-bit vs 64-bit).

500

Oh god, the large code model is so inefficient! Even the GOT offsets are potentially 64-bit. O_O

[aeubanks]

rebase, should address remaining comments

[rnk]

lgtm

llvm/lib/Target/X86/X86ISelLowering.cpp

unsigned X86TargetLowering::getGlobalWrapperKind(
   if (GV && GV->isAbsoluteSymbolRef())
     return X86ISD::Wrapper;
 
   CodeModel::Model M = getTargetMachine().getCodeModel();
   if (Subtarget.isPICStyleRIPRel() &&
       (M == CodeModel::Small || M == CodeModel::Kernel))
     return X86ISD::WrapperRIP;
 
   // In the medium model, functions can always be referenced RIP-relatively,
   // since they must be within 2GiB. This is also possible in non-PIC mode, and
   // shorter than the 64-bit absolute immediate that would otherwise be emitted.
-  if (M == CodeModel::Medium && isa_and_nonnull<Function>(GV))
+  // For PIC global variables, MO_NO_FLAG also means it can be referenced
+  // RIP-relatively, e.g. for data smaller than the large data threshold.
+  if (M == CodeModel::Medium &&
+      (isa_and_nonnull<Function>(GV) ||
+       (GV != nullptr && OpFlags == X86II::MO_NO_FLAG &&
+        Subtarget.isPICStyleRIPRel())))
     return X86ISD::WrapperRIP;
 
   // GOTPCREL references must always use RIP.
   if (OpFlags == X86II::MO_GOTPCREL || OpFlags == X86II::MO_GOTPCREL_NORELAX)
     return X86ISD::WrapperRIP;
 
   return X86ISD::Wrapper;
 }

llvm/lib/Target/X86/X86Subtarget.cpp

if (isTargetELF()) {
       case CodeModel::Small:
       case CodeModel::Kernel:
         return X86II::MO_NO_FLAG;
 
       // The large PIC code model uses GOTOFF.
       case CodeModel::Large:
         return X86II::MO_GOTOFF;
 
-      // Medium is a hybrid: RIP-rel for code, GOTOFF for DSO local data.
+      // Medium is a hybrid: RIP-rel for code and non-large data, GOTOFF for
+      // remaining DSO local data.
       case CodeModel::Medium:
         // Constant pool and jump table handling pass a nullptr to this
         // function so we need to use isa_and_nonnull.
         if (isa_and_nonnull<Function>(GV))
           return X86II::MO_NO_FLAG; // All code is RIP-relative
+        if (auto *GVar = dyn_cast_or_null<GlobalVariable>(GV)) {
+          if (!TM.isLargeData(GVar))
+            return X86II::MO_NO_FLAG;
+        }
         return X86II::MO_GOTOFF;    // Local symbols use GOTOFF.
       }
       llvm_unreachable("invalid code model");
     }
 
     // Otherwise, this is either a RIP-relative reference or a 64-bit movabsq,
     // both of which use MO_NO_FLAG.
     return X86II::MO_NO_FLAG;

llvm/test/CodeGen/X86/code-model-elf.ll

-; NOTE: Assertions have been autogenerated by utils/update_llc_test_checks.py
+; NOTE: Assertions have been autogenerated by utils/update_llc_test_checks.py UTC_ARGS: --no_x86_scrub_sp
 ; Run with --no_x86_scrub_rip because we care a lot about how globals are
 ; accessed in the code model.
 
 ; RUN: llc -verify-machineinstrs < %s -relocation-model=static -code-model=small  | FileCheck %s --check-prefix=CHECK --check-prefix=SMALL-STATIC
 ; RUN: llc -verify-machineinstrs < %s -relocation-model=static -code-model=medium | FileCheck %s --check-prefix=CHECK --check-prefix=MEDIUM-STATIC
 ; RUN: llc -verify-machineinstrs < %s -relocation-model=static -code-model=large  | FileCheck %s --check-prefix=CHECK --check-prefix=LARGE-STATIC
 ; RUN: llc -verify-machineinstrs < %s -relocation-model=pic    -code-model=small  | FileCheck %s --check-prefix=CHECK --check-prefix=SMALL-PIC
+; RUN: llc -verify-machineinstrs < %s -relocation-model=pic    -code-model=medium -large-data-threshold=1000 | FileCheck %s --check-prefix=CHECK --check-prefix=MEDIUM-SMALL-DATA-PIC
 ; RUN: llc -verify-machineinstrs < %s -relocation-model=pic    -code-model=medium | FileCheck %s --check-prefix=CHECK --check-prefix=MEDIUM-PIC
 ; RUN: llc -verify-machineinstrs < %s -relocation-model=pic    -code-model=large  | FileCheck %s --check-prefix=CHECK --check-prefix=LARGE-PIC
 
+; FIXME: small pic and medium pic w/ big enough large data threshold should be equivalent
+
 ; Generated from this C source:
 ;
 ; static int static_data[10];
 ; int global_data[10] = {1, 2};
 ; extern int extern_data[10];
 ;
 ; int *lea_static_data() { return &static_data[0]; }
 ; int *lea_global_data() { return &global_data[0]; }

 ; LARGE-STATIC-NEXT:    movabsq $static_data, %rax
 ; LARGE-STATIC-NEXT:    retq
 ;
 ; SMALL-PIC-LABEL: lea_static_data:
 ; SMALL-PIC:       # %bb.0:
 ; SMALL-PIC-NEXT:    leaq static_data(%rip), %rax
 ; SMALL-PIC-NEXT:    retq
 ;
+; MEDIUM-SMALL-DATA-PIC-LABEL: lea_static_data:
+; MEDIUM-SMALL-DATA-PIC:       # %bb.0:
+; MEDIUM-SMALL-DATA-PIC-NEXT:    leaq static_data(%rip), %rax
+; MEDIUM-SMALL-DATA-PIC-NEXT:    retq
+;
 ; MEDIUM-PIC-LABEL: lea_static_data:
 ; MEDIUM-PIC:       # %bb.0:
 ; MEDIUM-PIC-NEXT:    leaq _GLOBAL_OFFSET_TABLE_(%rip), %rcx
 ; MEDIUM-PIC-NEXT:    movabsq $static_data@GOTOFF, %rax
 ; MEDIUM-PIC-NEXT:    addq %rcx, %rax
 ; MEDIUM-PIC-NEXT:    retq
 ;
 ; LARGE-PIC-LABEL: lea_static_data:

 ; LARGE-STATIC-NEXT:    movabsq $global_data, %rax
 ; LARGE-STATIC-NEXT:    retq
 ;
 ; SMALL-PIC-LABEL: lea_global_data:
 ; SMALL-PIC:       # %bb.0:
 ; SMALL-PIC-NEXT:    leaq global_data(%rip), %rax
 ; SMALL-PIC-NEXT:    retq
 ;
+; MEDIUM-SMALL-DATA-PIC-LABEL: lea_global_data:
+; MEDIUM-SMALL-DATA-PIC:       # %bb.0:
+; MEDIUM-SMALL-DATA-PIC-NEXT:    leaq global_data(%rip), %rax
+; MEDIUM-SMALL-DATA-PIC-NEXT:    retq
+;
 ; MEDIUM-PIC-LABEL: lea_global_data:
 ; MEDIUM-PIC:       # %bb.0:
 ; MEDIUM-PIC-NEXT:    leaq _GLOBAL_OFFSET_TABLE_(%rip), %rcx
 ; MEDIUM-PIC-NEXT:    movabsq $global_data@GOTOFF, %rax
 ; MEDIUM-PIC-NEXT:    addq %rcx, %rax
 ; MEDIUM-PIC-NEXT:    retq
 ;
 ; LARGE-PIC-LABEL: lea_global_data:

 ; LARGE-STATIC-NEXT:    movabsq $extern_data, %rax
 ; LARGE-STATIC-NEXT:    retq
 ;
 ; SMALL-PIC-LABEL: lea_extern_data:
 ; SMALL-PIC:       # %bb.0:
 ; SMALL-PIC-NEXT:    movq extern_data@GOTPCREL(%rip), %rax
 ; SMALL-PIC-NEXT:    retq
 ;
+; MEDIUM-SMALL-DATA-PIC-LABEL: lea_extern_data:
+; MEDIUM-SMALL-DATA-PIC:       # %bb.0:
+; MEDIUM-SMALL-DATA-PIC-NEXT:    movq extern_data@GOTPCREL(%rip), %rax
+; MEDIUM-SMALL-DATA-PIC-NEXT:    retq
+;
 ; MEDIUM-PIC-LABEL: lea_extern_data:
 ; MEDIUM-PIC:       # %bb.0:
 ; MEDIUM-PIC-NEXT:    movq extern_data@GOTPCREL(%rip), %rax
 ; MEDIUM-PIC-NEXT:    retq
 ;
 ; LARGE-PIC-LABEL: lea_extern_data:
 ; LARGE-PIC:       # %bb.0:
 ; LARGE-PIC-NEXT:  .L2$pb:

 ; LARGE-STATIC-NEXT:    movl 8(%rax), %eax
 ; LARGE-STATIC-NEXT:    retq
 ;
 ; SMALL-PIC-LABEL: load_global_data:
 ; SMALL-PIC:       # %bb.0:
 ; SMALL-PIC-NEXT:    movl global_data+8(%rip), %eax
 ; SMALL-PIC-NEXT:    retq
 ;
+; MEDIUM-SMALL-DATA-PIC-LABEL: load_global_data:
+; MEDIUM-SMALL-DATA-PIC:       # %bb.0:
+; MEDIUM-SMALL-DATA-PIC-NEXT:    leaq global_data(%rip), %rax
+; MEDIUM-SMALL-DATA-PIC-NEXT:    movl 8(%rax), %eax
+; MEDIUM-SMALL-DATA-PIC-NEXT:    retq
+;
 ; MEDIUM-PIC-LABEL: load_global_data:
 ; MEDIUM-PIC:       # %bb.0:
 ; MEDIUM-PIC-NEXT:    leaq _GLOBAL_OFFSET_TABLE_(%rip), %rax
 ; MEDIUM-PIC-NEXT:    movabsq $global_data@GOTOFF, %rcx
 ; MEDIUM-PIC-NEXT:    movl 8(%rax,%rcx), %eax
 ; MEDIUM-PIC-NEXT:    retq
 ;
 ; LARGE-PIC-LABEL: load_global_data:

 ; LARGE-STATIC-NEXT:    retq
 ;
 ; SMALL-PIC-LABEL: load_extern_data:
 ; SMALL-PIC:       # %bb.0:
 ; SMALL-PIC-NEXT:    movq extern_data@GOTPCREL(%rip), %rax
 ; SMALL-PIC-NEXT:    movl 8(%rax), %eax
 ; SMALL-PIC-NEXT:    retq
 ;
+; MEDIUM-SMALL-DATA-PIC-LABEL: load_extern_data:
+; MEDIUM-SMALL-DATA-PIC:       # %bb.0:
+; MEDIUM-SMALL-DATA-PIC-NEXT:    movq extern_data@GOTPCREL(%rip), %rax
+; MEDIUM-SMALL-DATA-PIC-NEXT:    movl 8(%rax), %eax
+; MEDIUM-SMALL-DATA-PIC-NEXT:    retq
+;
 ; MEDIUM-PIC-LABEL: load_extern_data:
 ; MEDIUM-PIC:       # %bb.0:
 ; MEDIUM-PIC-NEXT:    movq extern_data@GOTPCREL(%rip), %rax
 ; MEDIUM-PIC-NEXT:    movl 8(%rax), %eax
 ; MEDIUM-PIC-NEXT:    retq
 ;
 ; LARGE-PIC-LABEL: load_extern_data:
 ; LARGE-PIC:       # %bb.0:

 define dso_local ptr @lea_static_fn() #0 {
 ; SMALL-STATIC-LABEL: lea_static_fn:
 ; SMALL-STATIC:       # %bb.0:
 ; SMALL-STATIC-NEXT:    movl $static_fn, %eax
 ; SMALL-STATIC-NEXT:    retq
 ;
 ; MEDIUM-STATIC-LABEL: lea_static_fn:
 ; MEDIUM-STATIC:       # %bb.0:
 ; MEDIUM-STATIC-NEXT:    leaq static_fn(%rip), %rax
 ; MEDIUM-STATIC-NEXT:    retq
 ;
 ; LARGE-STATIC-LABEL: lea_static_fn:
 ; LARGE-STATIC:       # %bb.0:
 ; LARGE-STATIC-NEXT:    movabsq $static_fn, %rax
 ; LARGE-STATIC-NEXT:    retq
 ;
 ; SMALL-PIC-LABEL: lea_static_fn:
 ; SMALL-PIC:       # %bb.0:
 ; SMALL-PIC-NEXT:    leaq static_fn(%rip), %rax
 ; SMALL-PIC-NEXT:    retq
 ;
+; MEDIUM-SMALL-DATA-PIC-LABEL: lea_static_fn:
+; MEDIUM-SMALL-DATA-PIC:       # %bb.0:
+; MEDIUM-SMALL-DATA-PIC-NEXT:    leaq static_fn(%rip), %rax
+; MEDIUM-SMALL-DATA-PIC-NEXT:    retq
+;
 ; MEDIUM-PIC-LABEL: lea_static_fn:
 ; MEDIUM-PIC:       # %bb.0:
 ; MEDIUM-PIC-NEXT:    leaq static_fn(%rip), %rax
 ; MEDIUM-PIC-NEXT:    retq
 ;
 ; LARGE-PIC-LABEL: lea_static_fn:
 ; LARGE-PIC:       # %bb.0:
 ; LARGE-PIC-NEXT:  .L7$pb:

 define dso_local ptr @lea_global_fn() #0 {
 ; SMALL-STATIC-LABEL: lea_global_fn:
 ; SMALL-STATIC:       # %bb.0:
 ; SMALL-STATIC-NEXT:    movl $global_fn, %eax
 ; SMALL-STATIC-NEXT:    retq
 ;
 ; MEDIUM-STATIC-LABEL: lea_global_fn:
 ; MEDIUM-STATIC:       # %bb.0:
 ; MEDIUM-STATIC-NEXT:    leaq global_fn(%rip), %rax
 ; MEDIUM-STATIC-NEXT:    retq
 ;
 ; LARGE-STATIC-LABEL: lea_global_fn:
 ; LARGE-STATIC:       # %bb.0:
 ; LARGE-STATIC-NEXT:    movabsq $global_fn, %rax
 ; LARGE-STATIC-NEXT:    retq
 ;
 ; SMALL-PIC-LABEL: lea_global_fn:
 ; SMALL-PIC:       # %bb.0:
 ; SMALL-PIC-NEXT:    leaq global_fn(%rip), %rax
 ; SMALL-PIC-NEXT:    retq
 ;
+; MEDIUM-SMALL-DATA-PIC-LABEL: lea_global_fn:
+; MEDIUM-SMALL-DATA-PIC:       # %bb.0:
+; MEDIUM-SMALL-DATA-PIC-NEXT:    leaq global_fn(%rip), %rax
+; MEDIUM-SMALL-DATA-PIC-NEXT:    retq
+;
 ; MEDIUM-PIC-LABEL: lea_global_fn:
 ; MEDIUM-PIC:       # %bb.0:
 ; MEDIUM-PIC-NEXT:    leaq global_fn(%rip), %rax
 ; MEDIUM-PIC-NEXT:    retq
 ;
 ; LARGE-PIC-LABEL: lea_global_fn:
 ; LARGE-PIC:       # %bb.0:
 ; LARGE-PIC-NEXT:  .L8$pb:

 ; LARGE-STATIC-NEXT:    movabsq $extern_fn, %rax
 ; LARGE-STATIC-NEXT:    retq
 ;
 ; SMALL-PIC-LABEL: lea_extern_fn:
 ; SMALL-PIC:       # %bb.0:
 ; SMALL-PIC-NEXT:    movq extern_fn@GOTPCREL(%rip), %rax
 ; SMALL-PIC-NEXT:    retq
 ;
+; MEDIUM-SMALL-DATA-PIC-LABEL: lea_extern_fn:
+; MEDIUM-SMALL-DATA-PIC:       # %bb.0:
+; MEDIUM-SMALL-DATA-PIC-NEXT:    movq extern_fn@GOTPCREL(%rip), %rax
+; MEDIUM-SMALL-DATA-PIC-NEXT:    retq
+;
 ; MEDIUM-PIC-LABEL: lea_extern_fn:
 ; MEDIUM-PIC:       # %bb.0:
 ; MEDIUM-PIC-NEXT:    movq extern_fn@GOTPCREL(%rip), %rax
 ; MEDIUM-PIC-NEXT:    retq
 ;
 ; LARGE-PIC-LABEL: lea_extern_fn:
 ; LARGE-PIC:       # %bb.0:
 ; LARGE-PIC-NEXT:  .L9$pb:

 ; LARGE-STATIC-NEXT:    addss (%rax), %xmm0
 ; LARGE-STATIC-NEXT:    retq
 ;
 ; SMALL-PIC-LABEL: load_constant_pool:
 ; SMALL-PIC:       # %bb.0:
 ; SMALL-PIC-NEXT:    addss {{\.?LCPI[0-9]+_[0-9]+}}(%rip), %xmm0
 ; SMALL-PIC-NEXT:    retq
 ;
+; MEDIUM-SMALL-DATA-PIC-LABEL: load_constant_pool:
+; MEDIUM-SMALL-DATA-PIC:       # %bb.0:
+; MEDIUM-SMALL-DATA-PIC-NEXT:    leaq _GLOBAL_OFFSET_TABLE_(%rip), %rax
+; MEDIUM-SMALL-DATA-PIC-NEXT:    movabsq ${{\.?LCPI[0-9]+_[0-9]+}}@GOTOFF, %rcx
+; MEDIUM-SMALL-DATA-PIC-NEXT:    addss (%rax,%rcx), %xmm0
+; MEDIUM-SMALL-DATA-PIC-NEXT:    retq
+;
 ; MEDIUM-PIC-LABEL: load_constant_pool:
 ; MEDIUM-PIC:       # %bb.0:
 ; MEDIUM-PIC-NEXT:    leaq _GLOBAL_OFFSET_TABLE_(%rip), %rax
 ; MEDIUM-PIC-NEXT:    movabsq ${{\.?LCPI[0-9]+_[0-9]+}}@GOTOFF, %rcx
 ; MEDIUM-PIC-NEXT:    addss (%rax,%rcx), %xmm0
 ; MEDIUM-PIC-NEXT:    retq
 ;
 ; LARGE-PIC-LABEL: load_constant_pool:
 ; LARGE-PIC:       # %bb.0:
 ; LARGE-PIC-NEXT:  .L11$pb:
 ; LARGE-PIC-NEXT:    leaq .L11$pb(%rip), %rax
 ; LARGE-PIC-NEXT:    movabsq $_GLOBAL_OFFSET_TABLE_-.L11$pb, %rcx
 ; LARGE-PIC-NEXT:    addq %rax, %rcx
 ; LARGE-PIC-NEXT:    movabsq ${{\.?LCPI[0-9]+_[0-9]+}}@GOTOFF, %rax
 ; LARGE-PIC-NEXT:    addss (%rcx,%rax), %xmm0
 ; LARGE-PIC-NEXT:    retq
   %a = fadd float %x, 1.0
   ret float %a
 }
 
 attributes #0 = { noinline nounwind uwtable }
 
 !llvm.module.flags = !{!0, !1, !2}
 !llvm.ident = !{!3}
 
 !0 = !{i32 1, !"wchar_size", i32 4}
 !1 = !{i32 7, !"PIC Level", i32 2}
 !2 = !{i32 7, !"PIE Level", i32 2}
 !3 = !{!"clang version 7.0.0 "}