[PowerPC] Change default for unaligned FP access for older subtargets

This is a fix for https://bugs.llvm.org/show_bug.cgi?id=40554

Some CPU's trap to the kernel on unaligned floating point access and there are
kernels that do not handle the interrupt. The program then fails with a SIGBUS
according to the PR. This just switches the default for unaligned access to only
allow it on recent server CPUs that are known to allow this.

Differential revision: https://reviews.llvm.org/D71954

3 files changed, 10 insertions, 1 deletions

diff --git a/llvm/lib/Target/PowerPC/PPC.td b/llvm/lib/Target/PowerPC/PPC.td
index 1d5396912ef..266b5bf1ba5 100644
--- a/llvm/lib/Target/PowerPC/PPC.td
+++ b/llvm/lib/Target/PowerPC/PPC.td
@@ -166,6 +166,9 @@ def FeatureHTM : SubtargetFeature<"htm", "HasHTM", "true",
                                   "Enable Hardware Transactional Memory instructions">;
 def FeatureMFTB   : SubtargetFeature<"", "FeatureMFTB", "true",
                                         "Implement mftb using the mfspr instruction">;
+def FeatureUnalignedFloats :
+  SubtargetFeature<"allow-unaligned-fp-access", "AllowsUnalignedFPAccess",
+                   "true", "CPU does not trap on unaligned FP access">;
 def FeaturePPCPreRASched:
   SubtargetFeature<"ppc-prera-sched", "UsePPCPreRASchedStrategy", "true",
                    "Use PowerPC pre-RA scheduling strategy">;
@@ -252,7 +255,8 @@ def ProcessorFeatures {
                                                   FeatureExtDiv,
                                                   FeatureMFTB,
                                                   DeprecatedDST,
-                                                  FeatureTwoConstNR];
+                                                  FeatureTwoConstNR,
+                                                  FeatureUnalignedFloats];
   list<SubtargetFeature> P7SpecificFeatures = [];
   list<SubtargetFeature> P7Features =
     !listconcat(P7InheritableFeatures, P7SpecificFeatures);
diff --git a/llvm/lib/Target/PowerPC/PPCISelLowering.cpp b/llvm/lib/Target/PowerPC/PPCISelLowering.cpp
index fa0b9a0b7af..e6969ca8750 100644
--- a/llvm/lib/Target/PowerPC/PPCISelLowering.cpp
+++ b/llvm/lib/Target/PowerPC/PPCISelLowering.cpp
@@ -15131,6 +15131,9 @@ bool PPCTargetLowering::allowsMisalignedMemoryAccesses(EVT VT,
   if (!VT.isSimple())
     return false;
 
+  if (VT.isFloatingPoint() && !Subtarget.allowsUnalignedFPAccess())
+    return false;
+
   if (VT.getSimpleVT().isVector()) {
     if (Subtarget.hasVSX()) {
       if (VT != MVT::v2f64 && VT != MVT::v2i64 &&
diff --git a/llvm/lib/Target/PowerPC/PPCSubtarget.h b/llvm/lib/Target/PowerPC/PPCSubtarget.h
index 6dff0c126ab..044e982740e 100644
--- a/llvm/lib/Target/PowerPC/PPCSubtarget.h
+++ b/llvm/lib/Target/PowerPC/PPCSubtarget.h
@@ -124,6 +124,7 @@ protected:
   bool IsPPC4xx;
   bool IsPPC6xx;
   bool FeatureMFTB;
+  bool AllowsUnalignedFPAccess;
   bool DeprecatedDST;
   bool HasLazyResolverStubs;
   bool IsLittleEndian;
@@ -274,6 +275,7 @@ public:
   bool vectorsUseTwoUnits() const {return VectorsUseTwoUnits; }
   bool isE500() const { return IsE500; }
   bool isFeatureMFTB() const { return FeatureMFTB; }
+  bool allowsUnalignedFPAccess() const { return AllowsUnalignedFPAccess; }
   bool isDeprecatedDST() const { return DeprecatedDST; }
   bool hasICBT() const { return HasICBT; }
   bool hasInvariantFunctionDescriptors() const {