[xray] Preserve flags in x86_64 trampolines.

Previously, some xray trampolines would modify condition codes (before saving/after restoring flags) due to stack alignment instructions, which use add/sub. I am not aware of issues that this causes in practice (outside of the situation described in #89364, which is only problematic due to a different bug). Nevertheless, it seems nicer and less error-prone for xray instrumentation to be as unobstrusive/preserve as much state as possible.

Author: rickyz

compiler-rt/lib/xray/xray_trampoline_x86_64.S

@@ -40,7 +40,7 @@
 CFI_ADJUST_CFA_OFFSET(-8)
 .endm
 
-// This macro should keep the stack aligned to 16 bytes.
+// This macro should lower the stack pointer by an odd multiple of 8.
 .macro SAVE_REGISTERS
 pushfq
 CFI_ADJUST_CFA_OFFSET(8)
@@ -70,7 +70,6 @@
 movq %r15, 0(%rsp)
 .endm
 
-// This macro should keep the stack aligned to 16 bytes.
 .macro RESTORE_REGISTERS
 movq 232(%rsp), %rbp
 movupd216(%rsp), %xmm0
@@ -117,8 +116,8 @@
 # LLVM-MCA-BEGIN __xray_FunctionEntry
 ASM_SYMBOL(__xray_FunctionEntry):
 CFI_STARTPROC
-ALIGN_STACK_16B
 SAVE_REGISTERS
+ALIGN_STACK_16B
 
 // This load has to be atomic, it's concurrent with __xray_patch().
 // On x86/amd64, a simple (type-aligned) MOV instruction is enough.
@@ -132,8 +131,8 @@ ASM_SYMBOL(__xray_FunctionEntry):
 callq*%rax
 
 LOCAL_LABEL(tmp0):
-RESTORE_REGISTERS
 RESTORE_STACK_ALIGNMENT
+RESTORE_REGISTERS
 retq
 # LLVM-MCA-END
 ASM_SIZE(__xray_FunctionEntry)
@@ -193,8 +192,8 @@ LOCAL_LABEL(tmp2):
 # LLVM-MCA-BEGIN __xray_FunctionTailExit
 ASM_SYMBOL(__xray_FunctionTailExit):
 CFI_STARTPROC
-ALIGN_STACK_16B
 SAVE_REGISTERS
+ALIGN_STACK_16B
 
 movqASM_SYMBOL(_ZN6__xray19XRayPatchedFunctionE)(%rip), %rax
 testq %rax,%rax
@@ -205,8 +204,8 @@ ASM_SYMBOL(__xray_FunctionTailExit):
 callq*%rax
 
 LOCAL_LABEL(tmp4):
-RESTORE_REGISTERS
 RESTORE_STACK_ALIGNMENT
+RESTORE_REGISTERS
 retq
 # LLVM-MCA-END
 ASM_SIZE(__xray_FunctionTailExit)
@@ -221,8 +220,8 @@ LOCAL_LABEL(tmp4):
 # LLVM-MCA-BEGIN __xray_ArgLoggerEntry
 ASM_SYMBOL(__xray_ArgLoggerEntry):
 CFI_STARTPROC
-ALIGN_STACK_16B
 SAVE_REGISTERS
+ALIGN_STACK_16B
 
 // Again, these function pointer loads must be atomic; MOV is fine.
 movqASM_SYMBOL(_ZN6__xray13XRayArgLoggerE)(%rip), %rax
@@ -248,8 +247,8 @@ LOCAL_LABEL(arg1entryLog):
 callq*%rax
 
 LOCAL_LABEL(arg1entryFail):
-RESTORE_REGISTERS
 RESTORE_STACK_ALIGNMENT
+RESTORE_REGISTERS
 retq
 # LLVM-MCA-END
 ASM_SIZE(__xray_ArgLoggerEntry)