[XRay][compiler-rt] Cleanup CFI/CFA annotations on trampolines

[dberris]

Summary

This is a follow-up to D32202.

While the previous change (D32202) did fix the stack alignment issue, we
were still at a weird state in terms of the CFI/CFA directives (as the
offsets were wrong). This change cleans up the SAVE/RESTORE macros for
the trampoline, accounting the stack pointer adjustments with less
instructions and with some clearer math. We note that the offsets will
be different on the exit trampolines, because we don't typically 'call'
into this trampoline and we only ever jump into them (i.e. treated as a
tail call that's patched in at runtime).

[kpw]

lib/xray/xray_trampoline_x86_64.S

104–105

If the exit handler is jumped into, does the exit sled explicitly push the return address onto the stack (accounting for the extra 8 bytes compared to the subq insn above)?

[pelikan]

Third time does the charm?

lib/xray/xray_trampoline_x86_64.S

104–105

Either the sled (unpatched) or this trampoline (patched) will execute the RET instruction which will pop the address from the stack. It will happen precisely once and therefore offset 8 is correct. The push happens before the entry into the function, and has nothing to do with XRay.

[dberris]

• fixup: update with a comment explaining the stack alignment expectations

[dberris]

Updated the macro with a comment explaining the situation for stack alignment (taking @eugenis' advice in D32202).

lib/xray/xray_trampoline_x86_64.S

 //
 // This implements the X86-specific assembler for the trampolines.
 //
 //===----------------------------------------------------------------------===//
 
 #include "../builtins/assembly.h"
 
 .macro SAVE_REGISTERS
-  subq $184, %rsp
+  subq $192, %rsp
+  .cfi_def_cfa_offset 200
+  // At this point, the stack pointer should be aligned to an 8-byte boundary,
+  // because any call instructions that come after this will add another 8
+  // bytes and therefore align it to 16-bytes.
+  movq %rbp, 184(%rsp)
   movupd  %xmm0, 168(%rsp)
   movupd  %xmm1, 152(%rsp)
   movupd  %xmm2, 136(%rsp)
   movupd  %xmm3, 120(%rsp)
   movupd  %xmm4, 104(%rsp)
   movupd  %xmm5, 88(%rsp)
   movupd  %xmm6, 72(%rsp)
   movupd  %xmm7, 56(%rsp)
   movq  %rdi, 48(%rsp)
   movq  %rax, 40(%rsp)
   movq  %rdx, 32(%rsp)
   movq  %rsi, 24(%rsp)
   movq  %rcx, 16(%rsp)
   movq  %r8, 8(%rsp)
   movq  %r9, 0(%rsp)
 .endm
 
 .macro RESTORE_REGISTERS
+  movq  184(%rsp), %rbp
   movupd  168(%rsp), %xmm0
   movupd  152(%rsp), %xmm1
   movupd  136(%rsp), %xmm2
   movupd  120(%rsp), %xmm3
   movupd  104(%rsp), %xmm4
   movupd  88(%rsp), %xmm5
   movupd  72(%rsp) , %xmm6
   movupd  56(%rsp) , %xmm7
   movq  48(%rsp), %rdi
   movq  40(%rsp), %rax
   movq  32(%rsp), %rdx
   movq  24(%rsp), %rsi
   movq  16(%rsp), %rcx
   movq  8(%rsp), %r8
   movq  0(%rsp), %r9
-  addq  $184, %rsp
+  addq  $192, %rsp
+  .cfi_def_cfa_offset 8
 .endm
 
   .text
   .file "xray_trampoline_x86.S"
 
 //===----------------------------------------------------------------------===//
 
   .globl __xray_FunctionEntry
   .align 16, 0x90
   .type __xray_FunctionEntry,@function
 
 __xray_FunctionEntry:
   .cfi_startproc
-  pushq %rbp
-  .cfi_def_cfa_offset 16
   SAVE_REGISTERS
-  .cfi_def_cfa_offset 200
 
   // This load has to be atomic, it's concurrent with __xray_patch().
   // On x86/amd64, a simple (type-aligned) MOV instruction is enough.
   movq  _ZN6__xray19XRayPatchedFunctionE(%rip), %rax
   testq %rax, %rax
   je  .Ltmp0
 
   // The patched function prolog puts its xray_instr_map index into %r10d.
   movl  %r10d, %edi
   xor %esi,%esi
   callq *%rax
 .Ltmp0:
   RESTORE_REGISTERS
-  popq  %rbp
   retq
 .Ltmp1:
   .size __xray_FunctionEntry, .Ltmp1-__xray_FunctionEntry
   .cfi_endproc
 
 //===----------------------------------------------------------------------===//
 
   .globl __xray_FunctionExit
   .align 16, 0x90
   .type __xray_FunctionExit,@function
 __xray_FunctionExit:
   .cfi_startproc
   // Save the important registers first. Since we're assuming that this
   // function is only jumped into, we only preserve the registers for
   // returning.
-  pushq %rbp
-  .cfi_def_cfa_offset 16
-  subq  $48, %rsp
+  subq  $56, %rsp
   .cfi_def_cfa_offset 64
+  movq  %rbp, 48(%rsp)
   movupd  %xmm0, 32(%rsp)
   movupd  %xmm1, 16(%rsp)
   movq  %rax, 8(%rsp)
   movq  %rdx, 0(%rsp)
   movq  _ZN6__xray19XRayPatchedFunctionE(%rip), %rax
   testq %rax,%rax
   je  .Ltmp2
 
   movl  %r10d, %edi
   movl  $1, %esi
   callq *%rax
 .Ltmp2:
   // Restore the important registers.
+  movq  48(%rsp), %rbp
   movupd  32(%rsp), %xmm0
   movupd  16(%rsp), %xmm1
   movq  8(%rsp), %rax
   movq  0(%rsp), %rdx
-  addq  $48, %rsp
-  popq  %rbp
+  addq  $56, %rsp
+  .cfi_def_cfa_offset 8
   retq
 .Ltmp3:
   .size __xray_FunctionExit, .Ltmp3-__xray_FunctionExit
   .cfi_endproc
 
 //===----------------------------------------------------------------------===//
 
   .global __xray_FunctionTailExit
   .align 16, 0x90
   .type __xray_FunctionTailExit,@function
 __xray_FunctionTailExit:
   .cfi_startproc
   // Save the important registers as in the entry trampoline, but indicate that
   // this is an exit. In the future, we will introduce a new entry type that
   // differentiates between a normal exit and a tail exit, but we'd have to do
   // this and increment the version number for the header.
-  pushq %rbp
-  .cfi_def_cfa_offset 16
   SAVE_REGISTERS
-  .cfi_def_cfa_offset 200
 
   movq  _ZN6__xray19XRayPatchedFunctionE(%rip), %rax
   testq %rax,%rax
   je  .Ltmp4
 
   movl  %r10d, %edi
   movl  $1, %esi
   callq *%rax
 
 .Ltmp4:
   RESTORE_REGISTERS
-  popq  %rbp
   retq
 .Ltmp5:
   .size __xray_FunctionTailExit, .Ltmp5-__xray_FunctionTailExit
   .cfi_endproc
 
 //===----------------------------------------------------------------------===//
 
   .globl __xray_ArgLoggerEntry
   .align 16, 0x90
   .type __xray_ArgLoggerEntry,@function
 __xray_ArgLoggerEntry:
   .cfi_startproc
-  pushq %rbp
-  .cfi_def_cfa_offset 16
   SAVE_REGISTERS
-  .cfi_def_cfa_offset 200
 
   // Again, these function pointer loads must be atomic; MOV is fine.
   movq  _ZN6__xray13XRayArgLoggerE(%rip), %rax
   testq %rax, %rax
   jne .Larg1entryLog
 
   // If [arg1 logging handler] not set, defer to no-arg logging.
   movq  _ZN6__xray19XRayPatchedFunctionE(%rip), %rax
   testq %rax, %rax
   je  .Larg1entryFail
 
 .Larg1entryLog:
   movq  %rdi, %rdx  // first argument will become the third
   xorq  %rsi, %rsi  // XRayEntryType::ENTRY into the second
   movl  %r10d, %edi  // 32-bit function ID becomes the first
   callq *%rax
 
 .Larg1entryFail:
   RESTORE_REGISTERS
-  popq  %rbp
   retq
 
 .Larg1entryEnd:
   .size __xray_ArgLoggerEntry, .Larg1entryEnd-__xray_ArgLoggerEntry
   .cfi_endproc
 
 NO_EXEC_STACK_DIRECTIVE