[BOLT] Enable --keep-nops option for Linux kernel by default (#86349)

Preserve nop instructions in the Linux kernel since they could be used
for runtime patching.

Author: maksfb

Diff for: bolt/lib/Rewrite/BinaryPassManager.cpp

@@ -72,7 +72,7 @@ static cl::opt<bool> JTFootprintReductionFlag(
              "instructions at jump sites"),
     cl::cat(BoltOptCategory));
 
-static cl::opt<bool>
+cl::opt<bool>
     KeepNops("keep-nops",
              cl::desc("keep no-op instructions. By default they are removed."),
              cl::Hidden, cl::cat(BoltOptCategory));

Diff for: bolt/lib/Rewrite/RewriteInstance.cpp

@@ -81,6 +81,7 @@ extern cl::list<std::string> HotTextMoveSections;
 extern cl::opt<bool> Hugify;
 extern cl::opt<bool> Instrument;
 extern cl::opt<JumpTableSupportLevel> JumpTables;
+extern cl::opt<bool> KeepNops;
 extern cl::list<std::string> ReorderData;
 extern cl::opt<bolt::ReorderFunctions::ReorderType> ReorderFunctions;
 extern cl::opt<bool> TimeBuild;
@@ -2031,6 +2032,9 @@ void RewriteInstance::adjustCommandLineOptions() {
 
   if (opts::Lite)
     BC->outs() << "BOLT-INFO: enabling lite mode\n";
+
+  if (BC->IsLinuxKernel && !opts::KeepNops.getNumOccurrences())
+    opts::KeepNops = true;
 }
 
 namespace {

Diff for: bolt/test/X86/linux-alt-instruction.s

@@ -6,31 +6,30 @@
 # RUN: llvm-mc -filetype=obj -triple x86_64-unknown-unknown %s -o %t.o
 # RUN: %clang %cflags -nostdlib %t.o -o %t.exe \
 # RUN:   -Wl,--image-base=0xffffffff80000000,--no-dynamic-linker,--no-eh-frame-hdr,--no-pie
-# RUN: llvm-bolt %t.exe --print-normalized --keep-nops -o %t.out \
-# RUN:   --alt-inst-feature-size=2 | FileCheck %s
+# RUN: llvm-bolt %t.exe --print-normalized --alt-inst-feature-size=2 -o %t.out \
+# RUN:   | FileCheck %s
 
 ## 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:   -Wl,--image-base=0xffffffff80000000,--no-dynamic-linker,--no-eh-frame-hdr,--no-pie
-# RUN: llvm-bolt %t.exe --print-normalized --keep-nops --alt-inst-has-padlen \
-# RUN:   -o %t.out | FileCheck %s
+# RUN: llvm-bolt %t.exe --print-normalized --alt-inst-has-padlen -o %t.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:   -Wl,--image-base=0xffffffff80000000,--no-dynamic-linker,--no-eh-frame-hdr,--no-pie
-# RUN: llvm-bolt %t.exe --print-normalized --keep-nops \
-# RUN:   --alt-inst-feature-size=4 -o %t.out | FileCheck %s
+# RUN: llvm-bolt %t.exe --print-normalized --alt-inst-feature-size=4 -o %t.out \
+# RUN:   | FileCheck %s
 
 ## Check that out-of-bounds read is handled properly.
 
-# RUN: not llvm-bolt %t.exe --print-normalized --keep-nops \
-# RUN:   --alt-inst-feature-size=2 -o %t.out
+# RUN: not llvm-bolt %t.exe --print-normalized --alt-inst-feature-size=2 -o %t.out
 
 # CHECK:      BOLT-INFO: Linux kernel binary detected
 # CHECK:      BOLT-INFO: parsed 2 alternative instruction entries

Diff for: bolt/test/X86/linux-orc.s

@@ -27,7 +27,7 @@
 ## Verify ORC bindings to instructions.
 
 # RUN: llvm-bolt %t.exe --print-normalized --dump-orc --print-orc -o %t.out \
-# RUN:   --bolt-info=0 |& FileCheck %s
+# RUN:   --keep-nops=0 --bolt-info=0 |& FileCheck %s
 
 
 ## Verify ORC bindings after rewrite.
@@ -37,7 +37,7 @@
 
 ## Verify ORC binding after rewrite when some of the functions are skipped.
 
-# RUN: llvm-bolt %t.exe -o %t.out --skip-funcs=bar --bolt-info=0
+# RUN: llvm-bolt %t.exe -o %t.out --skip-funcs=bar --bolt-info=0 --keep-nops=0
 # RUN: llvm-bolt %t.out -o %t.out.1 --print-normalized --print-orc \
 # RUN:   |& FileCheck %s
 

Diff for: bolt/test/X86/linux-parainstructions.s

@@ -8,7 +8,7 @@
 
 ## Verify paravirtual bindings to instructions.
 
-# RUN: llvm-bolt %t.exe --print-normalized -o %t.out | FileCheck %s
+# RUN: llvm-bolt %t.exe --print-normalized -o %t.out --keep-nops=0 | FileCheck %s
 
 # CHECK:      BOLT-INFO: Linux kernel binary detected
 # CHECK:      BOLT-INFO: parsed 2 paravirtual patch sites