[BOLT] Remove unreachable uncond branch after return

[yota9]

Summary

This patch fixes the removal of unreachable uncondtional branch located
after return instruction.

Vladislav Khmelevsky,
Advanced Software Technology Lab, Huawei

[Amir]

bolt/test/X86/jmp-after-ret.s

19

Are you sure 1f here means the label and not a hex offset? Can you please use .L1 instead to make it explicit?

[yota9]

bolt/test/X86/jmp-after-ret.s

19

This is default syntax for asm and called numeric label: https://docs.oracle.com/cd/E19120-01/open.solaris/817-5477/esqaq/index.html . I've checked the objdump for this (x86) and it works fine, and I'm sure about arm too :) I often see such syntax in linux kernel, so I think it is OK if you don't mind :)

[Amir]

bolt/test/X86/jmp-after-ret.s

19

Thanks for clarification. I wasn’t familiar with this syntax and it looks confusing, but as long as it works let’s keep it.

[Amir]

LGTM

bolt/lib/Core/BinaryFunction.cpp

if (!InsertBB) {
       // It must be a fallthrough or unreachable code. Create a new block unless
       // we see an unconditional branch following a conditional one. The latter
       // should not be a conditional tail call.
       assert(PrevBB && "no previous basic block for a fall through");
       MCInst *PrevInstr = PrevBB->getLastNonPseudoInstr();
       assert(PrevInstr && "no previous instruction for a fall through");
       if (MIB->isUnconditionalBranch(Instr) &&
           !MIB->isUnconditionalBranch(*PrevInstr) &&
-          !MIB->getConditionalTailCall(*PrevInstr)) {
+          !MIB->getConditionalTailCall(*PrevInstr) &&
+          !MIB->isReturn(*PrevInstr)) {
         // Temporarily restore inserter basic block.
         InsertBB = PrevBB;
       } else {
         MCSymbol *Label;
         {
           auto L = BC.scopeLock();
           Label = BC.Ctx->createNamedTempSymbol("FT");
         }

bolt/test/AArch64/jmp-after-ret.s

+## This test checks that the unreachable unconditional branch is removed
+## if it is located after return instruction.
+
+# REQUIRES: system-linux
+
+# RUN: llvm-mc -filetype=obj -triple aarch64-unknown-unknown \
+# RUN:   %s -o %t.o
+# RUN: %clang %cflags %t.o -o %t.exe -Wl,-q
+# RUN: llvm-bolt %t.exe -o %t.bolt | FileCheck %s
+
+# CHECK: UCE removed 1 blocks
+
+  .text
+  .align 4
+  .global main
+  .type main, %function
+main:
+  b.eq 1f
+  ret
+  b main
+1:
+  mov x1, #1
+  ret
+  .size main, .-main

bolt/test/X86/jmp-after-ret.s

+## This test checks that the unreachable unconditional branch is removed
+## if it is located after return instruction.
+
+# REQUIRES: system-linux
+
+# RUN: llvm-mc -filetype=obj -triple x86_64-unknown-unknown \
+# RUN:   %s -o %t.o
+# RUN: %clang %cflags %t.o -o %t.exe -Wl,-q
+# RUN: llvm-bolt %t.exe -o %t.bolt | FileCheck %s
+
+# CHECK: UCE removed 1 blocks
+
+    .text
+    .globl main
+    .type main, %function
+    .size main, .Lend-main
+main:
+    je 1f
+    retq
+    jmp main
+1:
+    movl $0x2, %ebx
+    retq
+.Lend: