llvmbot
commented
4 years ago Looks like the frontend slaps this line on the br:
define i32 @func_10() #0 !dbg !16 { %1 = alloca i32, align 8 %2 = alloca i32, align 4 %3 = alloca i32, align 4 %4 = bitcast i32 %1 to i8, !dbg !24 call void @llvm.lifetime.start.p0i8(i64 8, i8 %4) #3, !dbg !24 call void @llvm.dbg.declare(metadata i32 %1, metadata !20, metadata !DIExpression()), !dbg !25 store i32* @a, i32* %1, align 8, !dbg !25, !tbaa !26 %5 = bitcast i32 %2 to i8, !dbg !30 call void @llvm.lifetime.start.p0i8(i64 4, i8 %5) #3, !dbg !30 call void @llvm.dbg.declare(metadata i32 %2, metadata !21, metadata !DIExpression()), !dbg !31 %6 = bitcast i32 %3 to i8, !dbg !30 call void @llvm.lifetime.start.p0i8(i64 4, i8 %6) #3, !dbg !30 call void @llvm.dbg.declare(metadata i32* %3, metadata !22, metadata !DIExpression()), !dbg !32 br label %7, !dbg !30
[...]
!30 = !DILocation(line: 5, column: 3, scope: !16)
Extended Description
After setting a breakpoint at func_10 and running line 5 is hit. However, from disassembly, it seems that line 7 is being executed.
$ cat -n a.c 1 int a; 2 int b; 3 int func_10() { 4 int c = &a; 5 int i, j; 6 d: 7 if (c) 8 goto d; 9 i = 0; 10 for (; i;) 11 j = &c; 12 return c; 13 } 14 int main() { b = func_10(); }
$ cat a.c int a; int b; int func_10() { int c = &a; int i, j; d: if (c) goto d; i = 0; for (; i;) j = &c; return c; } int main() { b = func_10(); }
$ clang -v clang version 12.0.0 (https://github.com/llvm/llvm-project.git bc8be3054067ac822fc6d9f4f8e64c841f530f16) Target: x86_64-unknown-linux-gnu Thread model: posix InstalledDir: /usr/local/bin Found candidate GCC installation: /usr/lib/gcc/x86_64-linux-gnu/7 Found candidate GCC installation: /usr/lib/gcc/x86_64-linux-gnu/7.5.0 Found candidate GCC installation: /usr/lib/gcc/x86_64-linux-gnu/8 Selected GCC installation: /usr/lib/gcc/x86_64-linux-gnu/7.5.0 Candidate multilib: .;@m64 Selected multilib: .;@m64
$ clang -Og -g -o opt a.c
$ lldb opt lldb opt (lldb) target create "opt" Current executable set to opt' (x86_64). (lldb) b func_10 Breakpoint 1: where = opt`func_10 + 7 at a.c:5:3, address = 0x0000000000400487 (lldb) r Process 24746 launched: 'opt' (x86_64) Process 24746 stopped
func_10 at a.c:5:3 2 int *b; 3 int *func_10() { 4 int *c = &a; -> 5 int i, j; 6 d: 7 if (*c) 8 goto d; (lldb) di optfunc_10: 0x400480 <+0>: cmpl $0x0, 0x200ba9(%rip) ; opt.PT_LOAD[1]..bss + 7 -> 0x400487 <+7>: je 0x400492 ; <+18> at a.c:12:3 0x400489 <+9>: nopl (%rax) 0x400490 <+16>: jmp 0x400490 ; <+16> at a.c:7:7 0x400492 <+18>: movl $0x601030, %eax ; imm = 0x601030 0x400497 <+23>: retq(lldb)