[XRay][compiler-rt][x86_64] Preserve flags in x86_64 trampolines.

[rickyz]

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.

[llvmbot]

@llvm/pr-subscribers-xray

Author: Ricky Zhou (rickyz)

Changes

Previously, certain 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.

---

Full diff: https://github.com/llvm/llvm-project/pull/89452.diff

1 Files Affected:

• (modified) compiler-rt/lib/xray/xray_trampoline_x86_64.S (+7-8)

diff --git a/compiler-rt/lib/xray/xray_trampoline_x86_64.S b/compiler-rt/lib/xray/xray_trampoline_x86_64.S index ff3ac91071a60e..01098f60eeab8b 100644 --- a/compiler-rt/lib/xray/xray_trampoline_x86_64.S +++ b/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	movupd	216(%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	movq	ASM_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.	movq	ASM_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)