fixup! [BOLT] Add reading support for Linux kernel .altinstructions section

Author: maksfb

bolt/lib/Rewrite/LinuxKernelRewriter.cpp

@@ -39,7 +39,7 @@ static cl::opt<uint32_t>
 
 static cl::opt<bool>
     DumpAltInstructions("dump-alt-instructions",
-                        cl::desc("dump Linux alernative instructions info"),
+                        cl::desc("dump Linux alternative instructions info"),
                         cl::init(false), cl::Hidden, cl::cat(BoltCategory));
 
 static cl::opt<bool>

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

@@ -0,0 +1,92 @@
+# REQUIRES: system-linux
+
+## Check that BOLT correctly parses the Linux kernel .altinstructions section
+## and annotates alternative instructions.
+
+# 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
+
+## 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
+
+## 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
+
+# CHECK:      BOLT-INFO: Linux kernel binary detected
+# CHECK:      BOLT-INFO: parsed 2 alternative instruction entries
+
+  .text
+  .globl _start
+  .type _start, %function
+_start:
+# CHECK: Binary Function "_start"
+.L0:
+  rdtsc
+# CHECK:      rdtsc
+# CHECK-SAME: AltInst: 1
+# CHECK-SAME: AltInst2: 2
+  nop
+# CHECK-NEXT: nop
+# CHECK-SAME: AltInst: 1
+# CHECK-SAME: AltInst2: 2
+  nop
+  nop
+.L1:
+  ret
+  .size _start, .-_start
+
+  .section .altinstr_replacement,"ax",@progbits
+.A0:
+  lfence
+  rdtsc
+.A1:
+  rdtscp
+.Ae:
+
+## Alternative instruction info.
+  .section .altinstructions,"a",@progbits
+
+  .long .L0 - .   # org instruction
+  .long .A0 - .   # alt instruction
+.ifdef FEATURE_SIZE_4
+  .long 0x72      # feature flags
+.else
+  .word 0x72      # feature flags
+.endif
+  .byte .L1 - .L0 # org size
+  .byte .A1 - .A0 # alt size
+.ifdef PADLEN
+  .byte 0
+.endif
+
+  .long .L0 - .   # org instruction
+  .long .A1 - .   # alt instruction
+.ifdef FEATURE_SIZE_4
+  .long 0x3b      # feature flags
+.else
+  .word 0x3b      # feature flags
+.endif
+  .byte .L1 - .L0 # org size
+  .byte .Ae - .A1 # alt size
+.ifdef PADLEN
+  .byte 0
+.endif
+
+## Fake Linux Kernel sections.
+  .section __ksymtab,"a",@progbits
+  .section __ksymtab_gpl,"a",@progbits