[ARM] Recognize abi tag module flags

[paperchalice]

Recognize abi tag hints from frontend rather than from architecture and options.
Frontend part: #161106.

[llvmbot]

@llvm/pr-subscribers-backend-arm

Author: None (paperchalice)

Changes

Recognize abi tag hints from frontend rather than from architecture and options.

---

Full diff: https://github.com/llvm/llvm-project/pull/161306.diff

2 Files Affected:

• (modified) llvm/lib/Target/ARM/ARMAsmPrinter.cpp (+22-6)
• (added) llvm/test/CodeGen/ARM/build-attributes-module.ll (+14)

diff --git a/llvm/lib/Target/ARM/ARMAsmPrinter.cpp b/llvm/lib/Target/ARM/ARMAsmPrinter.cpp
index 1f773e2a7e0fc..125c002682012 100644
--- a/llvm/lib/Target/ARM/ARMAsmPrinter.cpp
+++ b/llvm/lib/Target/ARM/ARMAsmPrinter.cpp
@@ -701,9 +701,21 @@ void ARMAsmPrinter::emitAttributes() {
                       ARMBuildAttrs::AddressDirect);
   }
 
+  Metadata *MDEx = nullptr;
+  Metadata *MDDM = nullptr;
+  Metadata *MDNM = nullptr;
+  if (const Module *M = MMI->getModule()) {
+    MDEx = M->getModuleFlag("arm-eabi-fp-exceptions");
+    MDDM = M->getModuleFlag("arm-eabi-fp-denormal");
+    MDNM = M->getModuleFlag("arm-eabi-fp-number-model");
+  }
+
   // Set FP Denormals.
-  if (checkDenormalAttributeConsistency(*MMI->getModule(), "denormal-fp-math",
-                                        DenormalMode::getPreserveSign()))
+  if (auto *DM = mdconst::extract_or_null<ConstantInt>(MDDM))
+    ATS.emitAttribute(ARMBuildAttrs::ABI_FP_denormal, DM->getZExtValue());
+  else if (checkDenormalAttributeConsistency(*MMI->getModule(),
+                                             "denormal-fp-math",
+                                             DenormalMode::getPreserveSign()))
     ATS.emitAttribute(ARMBuildAttrs::ABI_FP_denormal,
                       ARMBuildAttrs::PreserveFPSign);
   else if (checkDenormalAttributeConsistency(*MMI->getModule(),
@@ -743,9 +755,11 @@ void ARMAsmPrinter::emitAttributes() {
   }
 
   // Set FP exceptions and rounding
-  if (checkFunctionsAttributeConsistency(*MMI->getModule(),
-                                         "no-trapping-math", "true") ||
-      TM.Options.NoTrappingFPMath)
+  if (auto *Ex = mdconst::extract_or_null<ConstantInt>(MDEx))
+    ATS.emitAttribute(ARMBuildAttrs::ABI_FP_exceptions, Ex->getZExtValue());
+  else if (checkFunctionsAttributeConsistency(*MMI->getModule(),
+                                              "no-trapping-math", "true") ||
+           TM.Options.NoTrappingFPMath)
     ATS.emitAttribute(ARMBuildAttrs::ABI_FP_exceptions,
                       ARMBuildAttrs::Not_Allowed);
   else {
@@ -759,7 +773,9 @@ void ARMAsmPrinter::emitAttributes() {
 
   // TM.Options.NoInfsFPMath && TM.Options.NoNaNsFPMath is the
   // equivalent of GCC's -ffinite-math-only flag.
-  if (TM.Options.NoInfsFPMath && TM.Options.NoNaNsFPMath)
+  if (auto *NM = mdconst::extract_or_null<ConstantInt>(MDNM))
+    ATS.emitAttribute(ARMBuildAttrs::ABI_FP_number_model, NM->getZExtValue());
+  else if (TM.Options.NoInfsFPMath && TM.Options.NoNaNsFPMath)
     ATS.emitAttribute(ARMBuildAttrs::ABI_FP_number_model,
                       ARMBuildAttrs::Allowed);
   else
diff --git a/llvm/test/CodeGen/ARM/build-attributes-module.ll b/llvm/test/CodeGen/ARM/build-attributes-module.ll
new file mode 100644
index 0000000000000..d7ac8e7df376f
--- /dev/null
+++ b/llvm/test/CodeGen/ARM/build-attributes-module.ll
@@ -0,0 +1,14 @@
+; RUN: llc %s -mtriple=arm-none-none-eabi -o - | FileCheck %s
+
+define void @f() {
+    ret void
+}
+
+!llvm.module.flags = !{!0, !1, !2}
+
+; CHECK: .eabi_attribute 20, 0
+!0 = !{i32 2, !"arm-eabi-fp-denormal", i32 0}
+; CHECK: .eabi_attribute 21, 1
+!1 = !{i32 2, !"arm-eabi-fp-exceptions", i32 1}
+; CHECK: .eabi_attribute 23, 1
+!2 = !{i32 2, !"arm-eabi-fp-number-model", i32 1}

[smithp35]

Just to make sure I understand. If there are no module flags for arm-eabi-fp-exceptions, arm-eabi-fp-denormal and arm-eabi-fp-number-model then the mdconst::extract_or_null<ConstantInt>(MD... ) will return nullptr and we'll fall back to checking function attribute consistency.

How are the module flags expected to be set by the front-end? In particular are the module flags always set (with default values) if the user does not express a preference.

I'd like to make sure that the LTO case where the user doesn't put the floating point options on the command line for the final link step (with just objects) still works as it does today.

[smithp35]

Is it ever possible for MMI->getModule() to be nullptr when writing build attributes? In the existing code for checkDenormalConsistency it is universally dereferenced?

If it isn't possible then I think it would be worth moving the M->getModuleFlag() calls closer to where they are used. I find it difficult to tell MDDM and MDNM apart with my eyesight.

For example (using SourceModule instead of M, as SourceModule is used on line 801).

const Module *SourceModule = MMI->getModule();
// Set FP Denormals
Metadata *MDDM = SourceModule->getModuleFlag("arm-eabi-fp-denormal");
if (auto *DM = mdconst::extract_or_null<ConstantInt>(MDDM))
  ATS.emitAttribute(ARMBuildAttrs::ABI_FP_denormal, DM->getZExtValue());
else if (checkDenormalAttributeConsistency(*SourceModule,
                                             "denormal-fp-math",
                                             DenormalMode::getPreserveSign()))

If we do have to preserve the if (const Module *M = MMI->getModule()) can you expand the MDDM and MDNM a bit? perhaps MDDenorm and MDNumModel .

[paperchalice]

How are the module flags expected to be set by the front-end? In particular are the module flags always set (with default values) if the user does not express a preference.

In #161106, arm-eabi-fp-denormal is 0 when -ffp-exception-behavior=ignore, arm-eabi-fp-number-model is controlled by -ffinite-math-only(quiet nan is not supported now)

I'd like to make sure that the LTO case where the user doesn't put the floating point options on the command line for the final link step (with just objects) still works as it does today.

Currently in pull request #161106 the behavior is warning, so there might be some extra warnings when these attributes are disagreed.