[BOLT] Add auto parsing for Linux kernel .altinstructions (#95068)

.altinstructions section contains a list of structures where fields can
have different sizes while other fields could be present or not
depending on the kernel version. Add automatic detection of such
variations and use it by default. The user can still overwrite the
automatic detection with `--alt-inst-has-padlen` and
`--alt-inst-feature-size` options.

Author: maksfb

bolt/lib/Rewrite/LinuxKernelRewriter.cpp

@@ -273,6 +273,8 @@ class LinuxKernelRewriter final : public MetadataRewriter {
 
   /// Handle alternative instruction info from .altinstructions.
   Error readAltInstructions();
+  Error tryReadAltInstructions(uint32_t AltInstFeatureSize,
+                               bool AltInstHasPadLen, bool ParseOnly);
   Error rewriteAltInstructions();
 
   /// Read .pci_fixup
@@ -1319,12 +1321,69 @@ Error LinuxKernelRewriter::rewriteBugTable() {
 ///	    u8  padlen;         // present in older kernels
 ///   } __packed;
 ///
-/// Note the structures is packed.
+/// Note that the structure is packed.
+///
+/// Since the size of the "feature" field could be either u16 or u32, and
+/// "padlen" presence is unknown, we attempt to parse .altinstructions section
+/// using all possible combinations (four at this time). Since we validate the
+/// contents of the section and its size, the detection works quite well.
+/// Still, we leave the user the opportunity to specify these features on the
+/// command line and skip the guesswork.
 Error LinuxKernelRewriter::readAltInstructions() {
   AltInstrSection = BC.getUniqueSectionByName(".altinstructions");
   if (!AltInstrSection)
     return Error::success();
 
+  // Presence of "padlen" field.
+  std::vector<bool> PadLenVariants;
+  if (opts::AltInstHasPadLen.getNumOccurrences())
+    PadLenVariants.push_back(opts::AltInstHasPadLen);
+  else
+    PadLenVariants = {false, true};
+
+  // Size (in bytes) variants of "feature" field.
+  std::vector<uint32_t> FeatureSizeVariants;
+  if (opts::AltInstFeatureSize.getNumOccurrences())
+    FeatureSizeVariants.push_back(opts::AltInstFeatureSize);
+  else
+    FeatureSizeVariants = {2, 4};
+
+  for (bool AltInstHasPadLen : PadLenVariants) {
+    for (uint32_t AltInstFeatureSize : FeatureSizeVariants) {
+      LLVM_DEBUG({
+        dbgs() << "BOLT-DEBUG: trying AltInstHasPadLen = " << AltInstHasPadLen
+               << "; AltInstFeatureSize = " << AltInstFeatureSize << ";\n";
+      });
+      if (Error E = tryReadAltInstructions(AltInstFeatureSize, AltInstHasPadLen,
+                                           /*ParseOnly*/ true)) {
+        consumeError(std::move(E));
+        continue;
+      }
+
+      LLVM_DEBUG(dbgs() << "Matched .altinstructions format\n");
+
+      if (!opts::AltInstHasPadLen.getNumOccurrences())
+        BC.outs() << "BOLT-INFO: setting --" << opts::AltInstHasPadLen.ArgStr
+                  << '=' << AltInstHasPadLen << '\n';
+
+      if (!opts::AltInstFeatureSize.getNumOccurrences())
+        BC.outs() << "BOLT-INFO: setting --" << opts::AltInstFeatureSize.ArgStr
+                  << '=' << AltInstFeatureSize << '\n';
+
+      return tryReadAltInstructions(AltInstFeatureSize, AltInstHasPadLen,
+                                    /*ParseOnly*/ false);
+    }
+  }
+
+  // We couldn't match the format. Read again to properly propagate the error
+  // to the user.
+  return tryReadAltInstructions(opts::AltInstFeatureSize,
+                                opts::AltInstHasPadLen, /*ParseOnly*/ false);
+}
+
+Error LinuxKernelRewriter::tryReadAltInstructions(uint32_t AltInstFeatureSize,
+                                                  bool AltInstHasPadLen,
+                                                  bool ParseOnly) {
   const uint64_t Address = AltInstrSection->getAddress();
   DataExtractor DE = DataExtractor(AltInstrSection->getContents(),
                                    BC.AsmInfo->isLittleEndian(),
@@ -1336,12 +1395,12 @@ Error LinuxKernelRewriter::readAltInstructions() {
         Address + Cursor.tell() + (int32_t)DE.getU32(Cursor);
     const uint64_t AltInstAddress =
         Address + Cursor.tell() + (int32_t)DE.getU32(Cursor);
-    const uint64_t Feature = DE.getUnsigned(Cursor, opts::AltInstFeatureSize);
+    const uint64_t Feature = DE.getUnsigned(Cursor, AltInstFeatureSize);
     const uint8_t OrgSize = DE.getU8(Cursor);
     const uint8_t AltSize = DE.getU8(Cursor);
 
     // Older kernels may have the padlen field.
-    const uint8_t PadLen = opts::AltInstHasPadLen ? DE.getU8(Cursor) : 0;
+    const uint8_t PadLen = AltInstHasPadLen ? DE.getU8(Cursor) : 0;
 
     if (!Cursor)
       return createStringError(
@@ -1358,7 +1417,7 @@ Error LinuxKernelRewriter::readAltInstructions() {
                 << "\n\tFeature: 0x" << Twine::utohexstr(Feature)
                 << "\n\tOrgSize: " << (int)OrgSize
                 << "\n\tAltSize: " << (int)AltSize << '\n';
-      if (opts::AltInstHasPadLen)
+      if (AltInstHasPadLen)
         BC.outs() << "\tPadLen:  " << (int)PadLen << '\n';
     }
 
@@ -1375,7 +1434,7 @@ Error LinuxKernelRewriter::readAltInstructions() {
 
     BinaryFunction *AltBF =
         BC.getBinaryFunctionContainingAddress(AltInstAddress);
-    if (AltBF && BC.shouldEmit(*AltBF)) {
+    if (!ParseOnly && AltBF && BC.shouldEmit(*AltBF)) {
       BC.errs()
           << "BOLT-WARNING: alternative instruction sequence found in function "
           << *AltBF << '\n';
@@ -1397,6 +1456,9 @@ Error LinuxKernelRewriter::readAltInstructions() {
                                " referenced by .altinstructions entry %d",
                                OrgInstAddress, EntryID);
 
+    if (ParseOnly)
+      continue;
+
     // There could be more than one alternative instruction sequences for the
     // same original instruction. Annotate each alternative separately.
     std::string AnnotationName = "AltInst";

bolt/test/X86/linux-alt-instruction.s

@@ -12,24 +12,30 @@
 ## Older kernels used to have padlen field in alt_instr. Check compatibility.
 
 # RUN: llvm-mc -filetype=obj -triple x86_64-unknown-unknown --defsym PADLEN=1 \
-# RUN:   %s -o %t.o
-# RUN: %clang %cflags -nostdlib %t.o -o %t.exe \
+# RUN:   %s -o %t.padlen.o
+# RUN: %clang %cflags -nostdlib %t.padlen.o -o %t.padlen.exe \
 # RUN:   -Wl,--image-base=0xffffffff80000000,--no-dynamic-linker,--no-eh-frame-hdr,--no-pie
-# RUN: llvm-bolt %t.exe --print-normalized --alt-inst-has-padlen -o %t.out \
+# RUN: llvm-bolt %t.padlen.exe --print-normalized --alt-inst-has-padlen -o %t.padlen.out \
 # RUN:   | FileCheck %s
 
 ## Check with a larger size of "feature" field in alt_instr.
 
 # RUN: llvm-mc -filetype=obj -triple x86_64-unknown-unknown \
-# RUN:   --defsym FEATURE_SIZE_4=1 %s -o %t.o
-# RUN: %clang %cflags -nostdlib %t.o -o %t.exe \
+# RUN:   --defsym FEATURE_SIZE_4=1 %s -o %t.fs4.o
+# RUN: %clang %cflags -nostdlib %t.fs4.o -o %t.fs4.exe \
 # RUN:   -Wl,--image-base=0xffffffff80000000,--no-dynamic-linker,--no-eh-frame-hdr,--no-pie
-# RUN: llvm-bolt %t.exe --print-normalized --alt-inst-feature-size=4 -o %t.out \
+# RUN: llvm-bolt %t.fs4.exe --print-normalized --alt-inst-feature-size=4 -o %t.fs4.out \
 # RUN:   | FileCheck %s
 
 ## Check that out-of-bounds read is handled properly.
 
-# RUN: not llvm-bolt %t.exe --print-normalized --alt-inst-feature-size=2 -o %t.out
+# RUN: not llvm-bolt %t.fs4.exe --alt-inst-feature-size=2 -o %t.fs4.out
+
+## Check that BOLT automatically detects structure fields in .altinstructions.
+
+# RUN: llvm-bolt %t.exe --print-normalized -o %t.out | FileCheck %s
+# RUN: llvm-bolt %t.exe --print-normalized -o %t.padlen.out | FileCheck %s
+# RUN: llvm-bolt %t.exe --print-normalized -o %t.fs4.out | FileCheck %s
 
 # CHECK:      BOLT-INFO: Linux kernel binary detected
 # CHECK:      BOLT-INFO: parsed 2 alternative instruction entries