通过修改LLVM后端注册Clobber X86 [英] Clobber X86 register by modifying LLVM Backend
问题描述
我正在尝试对X86目标的LLVM后端进行一些更改，以产生一些所需的行为.
I am trying to alter a little bit the LLVM Backend for X86 target, to produce some desired behaviour.
更具体地说，我想模拟gcc的fcall-used- reg 这样的标记
More specifically, I would like to emulate a flag like gcc's fcall-used-reg option, which instructs the compiler to convert a callee-saved register into a clobbered register (meaning that it may be altered during a function call).
让我们专注于 r14 .我手动破坏寄存器，例如这个答案:
Let's focus on r14. I manually clobber the register, like in this answer:
#include <inttypes.h>
uint64_t inc(uint64_t i) {
__asm__ __volatile__(
""
: "+m" (i)
:
: "r14"
);
return i + 1;
}
int main(int argc, char **argv) {
(void)argv;
return inc(argc);
}
编译和反汇编:
gcc -std=gnu99 -O3 -ggdb3 -Wall -Wextra -pedantic -o main.out main.c
objdump -d main.out
反汇编包含:
0000000000001150 <inc>:
1150: 41 56 push %r14
1152: 48 89 7c 24 f8 mov %rdi,-0x8(%rsp)
1157: 48 8b 44 24 f8 mov -0x8(%rsp),%rax
115c: 41 5e pop %r14
115e: 48 83 c0 01 add $0x1,%rax
1162: c3 retq
1163: 66 2e 0f 1f 84 00 00 nopw %cs:0x0(%rax,%rax,1)
116a: 00 00 00
116d: 0f 1f 00 nopl (%rax)
我们可以看到 r14 因为被篡改而被压入堆栈，然后弹出以恢复其原始值.现在，重复 -fcall-used-r14 标志:
where we can see that r14, because it is tampered with, is pushed to the stack, and then popped to regain its original value. Now, repeat with the -fcall-used-r14 flag:
gcc -std=gnu99 -O3 -ggdb3 -fcall-used-r14 -Wall -Wextra -pedantic -o main.out main.c
objdump -d main.out
反汇编包含:
0000000000001150 <inc>:
1150: 48 89 7c 24 f8 mov %rdi,-0x8(%rsp)
1155: 48 8b 44 24 f8 mov -0x8(%rsp),%rax
115a: 48 83 c0 01 add $0x1,%rax
115e: c3 retq
115f: 90 nop
没有推送/弹出的地方.
where no push/pop happens.
现在，我已经修改了一些LLVM目标文件，编译了源文件，并将此功能添加到了 llc 工具中:
Now, I have modified some LLVM Target files, compiled the source, and added(?) this functionality to the llc tool:
clang-11 -emit-llvm -S -c main.c -o main.ll
llc-11 main.ll -o main.s
现在， main.s 包含:
# %bb.0:
pushq %rbp
.cfi_def_cfa_offset 16
.cfi_offset %rbp, -16
movq %rsp, %rbp
.cfi_def_cfa_register %rbp
pushq %r14
.cfi_offset %r14, -24
movq %rdi, -16(%rbp)
#APP
#NO_APP
movq -16(%rbp), %rax
addq $1, %rax
popq %r14
popq %rbp
.cfi_def_cfa %rsp, 8
retq
显然， r14 仍然被呼叫者保存.
Apparently, r14 is still callee-saved.
在 llvm/lib/Target/X86/X86CallingConv.td 内部，我修改了以下几行(删除了R14)，因为它们似乎与Linux和C调用约定的System V ABI唯一相关我感兴趣的:
Inside llvm/lib/Target/X86/X86CallingConv.td I have modified the following lines (removing R14), because they seemed the only relevant to the System V ABI for Linux and C calling conventions that I was interested in:
def CSR_64 : CalleeSavedRegs<(add R12, R13, R15, RBP)>;
...
def CSR_64_MostRegs : CalleeSavedRegs<(add RBX, RCX, RDX, RSI, RDI, R8, R9, R10,
R11, R12, R13, R15, RBP,
...
def CSR_64_AllRegs_NoSSE : CalleeSavedRegs<(add RAX, RBX, RCX, RDX, RSI, RDI, R8, R9,
R10, R11, R12, R13, R15, RBP)>;
我的问题是:
-
X86CallingConv.td是我唯一需要修改的文件吗?我想是的，但也许我错了. - 我专注于正确的路线吗?也许这很难回答，但至少一个方向可能会有所帮助.
- Is
X86CallingConv.tdthe only file I should modify? I think yes, but maybe I'm wrong. - Am I focusing on the correct lines? Maybe this is more difficult to answer, but at least a direction could be helpful.
我正在Debian 10.5中运行LLVM 11.
I am running LLVM 11 inside Debian 10.5.
改变线，从隐藏的" R 14中除去R 14.定义:
Changing the line, removing R14 from "hidden" definition:
def CSR_SysV64_RegCall_NoSSE : CalleeSavedRegs<(add RBX, RBP, RSP,
(sequence "R%u", 12, 13), R15)>;
正如玛格丽特正确指出的那样，也无济于事.
as Margaret correctly pointed out did not help either.
推荐答案
结果是，最小的修改是这一行:
Turns out, the minimum modification was the line:
def CSR_64 : CalleeSavedRegs<(add RBX, R12, R13, R15, RBP)>;
问题出在我如何构建源代码上.
The problem was with how I built the source.
通过在原始安装后再次运行 cmake --build ， llc 工具未进行全局修改(我认为这样做是因为我正在构建默认体系结构-X86-但这无关紧要).因此，我打电话给了未经修改的 llc-11 工具.因此，当我跑步时:
By running cmake --build . again after the original installation, the llc tool was not modified globally (I thought it would have because I was building the default architecture - X86 - but that was irrelevant). So, I was calling an unmodified llc-11 tool. Thus, when I ran:
/path/to/llvm-project/build/bin/lcc main.ll -o main.s
main.s 包含:
# %bb.0:
movq %rdi, -8(%rsp)
#APP
#NO_APP
movq -8(%rsp), %rax
addq $1, %rax
retq
这是我首先要的.
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