[X86] Place data in large sections for large code model

[aeubanks]

This allows better interoperability mixing small/medium/large code model
code since large code model data can be put into separate large sections.

And respect large data threshold under large code model.
gcc also does this: https://gcc.gnu.org/onlinedocs/gcc/x86-Options.html.

See https://groups.google.com/g/x86-64-abi/c/jnQdJeabxiU.

[llvmbot]

@llvm/pr-subscribers-backend-x86

Author: Arthur Eubanks (aeubanks)

Changes

This allows better interoperability mixing small/medium/large code model
code since small code model data won't be mixed with large code model
data.

---

Full diff: https://github.com/llvm/llvm-project/pull/70265.diff

2 Files Affected:

• (modified) llvm/lib/Target/TargetMachine.cpp (+13-6)
• (modified) llvm/test/CodeGen/X86/code-model-elf-sections.ll (+5-2)

diff --git a/llvm/lib/Target/TargetMachine.cpp b/llvm/lib/Target/TargetMachine.cpp
index 45fb612cb91da19..e11355de2eed3b5 100644
--- a/llvm/lib/Target/TargetMachine.cpp
+++ b/llvm/lib/Target/TargetMachine.cpp
@@ -42,13 +42,20 @@ TargetMachine::~TargetMachine() = default;
 bool TargetMachine::isLargeData(const GlobalVariable *GV) const {
   if (getTargetTriple().getArch() != Triple::x86_64 || GV->isThreadLocal())
     return false;
-  // Large data under the large code model still needs to be thought about, so
-  // restrict this to medium.
-  if (getCodeModel() != CodeModel::Medium)
+
+  switch (getCodeModel()) {
+  case CodeModel::Large: {
+    return true;
+  }
+  case CodeModel::Medium: {
+    const DataLayout &DL = GV->getParent()->getDataLayout();
+    uint64_t Size = DL.getTypeSizeInBits(GV->getValueType()) / 8;
+    return Size == 0 || Size > LargeDataThreshold;
+  }
+  default: {
     return false;
-  const DataLayout &DL = GV->getParent()->getDataLayout();
-  uint64_t Size = DL.getTypeSizeInBits(GV->getValueType()) / 8;
-  return Size == 0 || Size > LargeDataThreshold;
+  }
+  }
 }
 
 bool TargetMachine::isPositionIndependent() const {
diff --git a/llvm/test/CodeGen/X86/code-model-elf-sections.ll b/llvm/test/CodeGen/X86/code-model-elf-sections.ll
index fe659fa9a46e727..5d02fed05814039 100644
--- a/llvm/test/CodeGen/X86/code-model-elf-sections.ll
+++ b/llvm/test/CodeGen/X86/code-model-elf-sections.ll
@@ -7,14 +7,17 @@
 ; RUN: llc < %s -relocation-model=pic -filetype=obj -code-model=medium -large-data-threshold=80 -o %t
 ; RUN: llvm-readelf -S %t | FileCheck %s --check-prefix=SMALL
 ; RUN: llc < %s -relocation-model=pic -filetype=obj -code-model=large -o %t
-; RUN: llvm-readelf -S %t | FileCheck %s --check-prefix=SMALL
+; RUN: llvm-readelf -S %t | FileCheck %s --check-prefix=LARGE
+; Check that the large code model ignores -large-data-threshold.
+; RUN: llc < %s -relocation-model=pic -filetype=obj -code-model=large -large-data-threshold=800 -o %t
+; RUN: llvm-readelf -S %t | FileCheck %s --check-prefix=LARGE
 
 ; RUN: llc < %s -relocation-model=pic -filetype=obj -code-model=small -data-sections -o %t
 ; RUN: llvm-readelf -S %t | FileCheck %s --check-prefix=SMALL-DS
 ; RUN: llc < %s -relocation-model=pic -filetype=obj -code-model=medium -data-sections -o %t
 ; RUN: llvm-readelf -S %t | FileCheck %s --check-prefix=LARGE-DS
 ; RUN: llc < %s -relocation-model=pic -filetype=obj -code-model=large -data-sections -o %t
-; RUN: llvm-readelf -S %t | FileCheck %s --check-prefix=SMALL-DS
+; RUN: llvm-readelf -S %t | FileCheck %s --check-prefix=LARGE-DS
 
 ; SMALL: .data {{.*}} WA {{.*}}
 ; SMALL: foo {{.*}} WA {{.*}}

[MaskRay]

I have recently changed GCC https://gcc.gnu.org/onlinedocs/gcc/x86-Options.html
-mcmodel=large also respects -mlarge-data-threshold=

[MaskRay]

Consider adding a link to https://groups.google.com/g/x86-64-abi/c/jnQdJeabxiU

[aeubanks]

I'm still of the opinion that large code model data should never contribute toward relocation pressure, but I'll follow gcc here.

[aeubanks]

ping

[rnk]

What controls the default for the large data size threshold, under the large code model? I think we discussed making it zero, so effectively all globals will be large data under the large code model, which sounds like the right behavior. I don't know which way GCC went on that.

I think we want to keep the .bss/.data/.rodata prefix checks, even under the large code model. I expect folks to mix large and small code, and we don't want to accidentally make small sections large.

[aeubanks]

we're relying on the frontend to set the large data threshold in TargetMachine, otherwise it's just 0

gcc currently defaults to 65536 as the large data threshold for the large code model

[MaskRay]

I'm still of the opinion that large code model data should never contribute toward relocation pressure, but I'll follow gcc here.

The decision is deliberate. Some users may expect .data.* section name and build features on top of it. These users (say JIT programs) may not care about he small/large code model mixing story.
-mlarge-data-threshold= gives users choices.