[libc-commits] [libc] [libc][setjmp] make x86_64 longjmp naked (PR #112581)
Nick Desaulniers via libc-commits
libc-commits at lists.llvm.org
Wed Oct 16 14:00:54 PDT 2024
https://github.com/nickdesaulniers updated https://github.com/llvm/llvm-project/pull/112581
>From 6ca41d94bed4c2f78495bbf40d602fd8c5ea7566 Mon Sep 17 00:00:00 2001
From: Nick Desaulniers <ndesaulniers at google.com>
Date: Wed, 16 Oct 2024 09:59:01 -0700
Subject: [PATCH 1/2] [libc][setjmp] make x86_64 longjmp naked
The generated asm for x86_64's longjmp has a function prolog and epilog. The
epilog in particular is unreachable. Convert longjmp to a naked function to
avoid these spurious instructions in longjmp.
Link: https://github.com/llvm/llvm-project/pull/112437/files#r1802085511
---
libc/src/setjmp/longjmp.h | 13 ++++++++
libc/src/setjmp/x86_64/longjmp.cpp | 48 ++++++++++++++++--------------
2 files changed, 38 insertions(+), 23 deletions(-)
diff --git a/libc/src/setjmp/longjmp.h b/libc/src/setjmp/longjmp.h
index 7cb12b3392ae16..9b7db29717216b 100644
--- a/libc/src/setjmp/longjmp.h
+++ b/libc/src/setjmp/longjmp.h
@@ -11,9 +11,22 @@
#include "hdr/types/jmp_buf.h"
#include "src/__support/macros/config.h"
+#include "src/__support/macros/properties/compiler.h"
namespace LIBC_NAMESPACE_DECL {
+// TODO(https://github.com/llvm/llvm-project/issues/112427)
+// Some of the architecture-specific definitions are marked `naked`, which in
+// GCC implies `nothrow`.
+//
+// Right now, our aliases aren't marked `nothrow`, so we wind up in a situation
+// where clang will emit -Wmissing-exception-spec if we add `nothrow` here, but
+// GCC will emit -Wmissing-attributes here without `nothrow`. We need to update
+// LLVM_LIBC_FUNCTION to denote when a function throws or not.
+
+#ifdef LIBC_COMPILER_IS_GCC
+[[gnu::nothrow]]
+#endif
void longjmp(jmp_buf buf, int val);
} // namespace LIBC_NAMESPACE_DECL
diff --git a/libc/src/setjmp/x86_64/longjmp.cpp b/libc/src/setjmp/x86_64/longjmp.cpp
index d4b55565cb2187..5d5da673348ede 100644
--- a/libc/src/setjmp/x86_64/longjmp.cpp
+++ b/libc/src/setjmp/x86_64/longjmp.cpp
@@ -7,6 +7,7 @@
//===----------------------------------------------------------------------===//
#include "src/setjmp/longjmp.h"
+#include "include/llvm-libc-macros/offsetof-macro.h"
#include "src/__support/common.h"
#include "src/__support/macros/config.h"
@@ -16,30 +17,31 @@
namespace LIBC_NAMESPACE_DECL {
-LLVM_LIBC_FUNCTION(void, longjmp, (jmp_buf buf, int val)) {
- register __UINT64_TYPE__ rbx __asm__("rbx");
- register __UINT64_TYPE__ rbp __asm__("rbp");
- register __UINT64_TYPE__ r12 __asm__("r12");
- register __UINT64_TYPE__ r13 __asm__("r13");
- register __UINT64_TYPE__ r14 __asm__("r14");
- register __UINT64_TYPE__ r15 __asm__("r15");
- register __UINT64_TYPE__ rsp __asm__("rsp");
- register __UINT64_TYPE__ rax __asm__("rax");
+[[gnu::naked]]
+LLVM_LIBC_FUNCTION(void, longjmp, (jmp_buf, int)) {
+ asm(R"(
+ cmpl $0x1, %%esi
+ adcl $0x0, %%esi
+ movq %%rsi, %%rax
- // ABI requires that the return value should be stored in rax. So, we store
- // |val| in rax. Note that this has to happen before we restore the registers
- // from values in |buf|. Otherwise, once rsp and rbp are updated, we cannot
- // read |val|.
- val = val == 0 ? 1 : val;
- LIBC_INLINE_ASM("mov %1, %0\n\t" : "=r"(rax) : "m"(val) :);
- LIBC_INLINE_ASM("mov %1, %0\n\t" : "=r"(rbx) : "m"(buf->rbx) :);
- LIBC_INLINE_ASM("mov %1, %0\n\t" : "=r"(rbp) : "m"(buf->rbp) :);
- LIBC_INLINE_ASM("mov %1, %0\n\t" : "=r"(r12) : "m"(buf->r12) :);
- LIBC_INLINE_ASM("mov %1, %0\n\t" : "=r"(r13) : "m"(buf->r13) :);
- LIBC_INLINE_ASM("mov %1, %0\n\t" : "=r"(r14) : "m"(buf->r14) :);
- LIBC_INLINE_ASM("mov %1, %0\n\t" : "=r"(r15) : "m"(buf->r15) :);
- LIBC_INLINE_ASM("mov %1, %0\n\t" : "=r"(rsp) : "m"(buf->rsp) :);
- LIBC_INLINE_ASM("jmp *%0\n\t" : : "m"(buf->rip));
+ movq %c[rbx](%%rdi), %%rbx
+ movq %c[rbp](%%rdi), %%rbp
+ movq %c[r12](%%rdi), %%r12
+ movq %c[r13](%%rdi), %%r13
+ movq %c[r14](%%rdi), %%r14
+ movq %c[r15](%%rdi), %%r15
+ movq %c[rsp](%%rdi), %%rsp
+ jmpq *%c[rip](%%rdi)
+ )" ::
+ [rbx] "i"(offsetof(__jmp_buf, rbx)),
+ [rbp] "i"(offsetof(__jmp_buf, rbp)),
+ [r12] "i"(offsetof(__jmp_buf, r12)),
+ [r13] "i"(offsetof(__jmp_buf, r13)),
+ [r14] "i"(offsetof(__jmp_buf, r14)),
+ [r15] "i"(offsetof(__jmp_buf, r15)),
+ [rsp] "i"(offsetof(__jmp_buf, rsp)),
+ [rip] "i"(offsetof(__jmp_buf, rip))
+);
}
} // namespace LIBC_NAMESPACE_DECL
>From 99080665ccac9d2425d100fb1a1bf182352ec306 Mon Sep 17 00:00:00 2001
From: Nick Desaulniers <ndesaulniers at google.com>
Date: Wed, 16 Oct 2024 14:00:41 -0700
Subject: [PATCH 2/2] reformat
---
libc/src/setjmp/x86_64/longjmp.cpp | 15 +++++----------
1 file changed, 5 insertions(+), 10 deletions(-)
diff --git a/libc/src/setjmp/x86_64/longjmp.cpp b/libc/src/setjmp/x86_64/longjmp.cpp
index 5d5da673348ede..c293c55a6f9fb2 100644
--- a/libc/src/setjmp/x86_64/longjmp.cpp
+++ b/libc/src/setjmp/x86_64/longjmp.cpp
@@ -32,16 +32,11 @@ LLVM_LIBC_FUNCTION(void, longjmp, (jmp_buf, int)) {
movq %c[r15](%%rdi), %%r15
movq %c[rsp](%%rdi), %%rsp
jmpq *%c[rip](%%rdi)
- )" ::
- [rbx] "i"(offsetof(__jmp_buf, rbx)),
- [rbp] "i"(offsetof(__jmp_buf, rbp)),
- [r12] "i"(offsetof(__jmp_buf, r12)),
- [r13] "i"(offsetof(__jmp_buf, r13)),
- [r14] "i"(offsetof(__jmp_buf, r14)),
- [r15] "i"(offsetof(__jmp_buf, r15)),
- [rsp] "i"(offsetof(__jmp_buf, rsp)),
- [rip] "i"(offsetof(__jmp_buf, rip))
-);
+ )" ::[rbx] "i"(offsetof(__jmp_buf, rbx)),
+ [rbp] "i"(offsetof(__jmp_buf, rbp)), [r12] "i"(offsetof(__jmp_buf, r12)),
+ [r13] "i"(offsetof(__jmp_buf, r13)), [r14] "i"(offsetof(__jmp_buf, r14)),
+ [r15] "i"(offsetof(__jmp_buf, r15)), [rsp] "i"(offsetof(__jmp_buf, rsp)),
+ [rip] "i"(offsetof(__jmp_buf, rip)));
}
} // namespace LIBC_NAMESPACE_DECL
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