[PATCH] D93983: RegAllocFast: Do not free later early-clobbered registers.

[LuoYuanke via Phabricator via llvm-commits]

LuoYuanke added inline comments.


================
Comment at: llvm/test/CodeGen/X86/phys-msInline-fastregalloc.ll:11
+  %m32 = bitcast { [4 x i32] }* %c to [4 x i32]*
+  %0 = call i32 asm sideeffect inteldialect "mov eax,$$1\0A\09mov $0,eax", "=*m,={eax},~{eax},~{dirflag},~{fpsr},~{flags}"([4 x i32]* %m32) #1, !srcloc !3
+  store i32 %0, i32* %retval, align 4
----------------
It seems to be invalid inline assembly. We can't set eax both as output and clobber. The clobbered register and output register should be in different physical register.

```
[llvm]$ cat t.c

void foo(int a, int b) {
  int c;
  asm volatile("addl %1, %2 \n"
               "movl %2, %0\n"
               :"=a"(c)
               :"r"(a), "r"(b)
               :"eax");
}
[llvm]$ clang -S t.c
t.c:8:17: error: asm-specifier for input or output variable conflicts with asm clobber list
               :"eax");
                ^
1 error generated.
[llvm]$ gcc -S t.c
t.c: In function ‘foo’:
t.c:4:3: error: ‘asm’ operand has impossible constraints
    4 |   asm volatile("addl %1, %2 \n"
      |   ^~~
[llvm]$
```

This is from https://llvm.org/docs/LangRef.html#output-constraints.

```
If this is not safe (e.g. if the assembly contains two instructions, where the first writes to one output, and the second reads an input and writes to a second output), then the “&” modifier must be used (e.g. “=&r”) to specify that the output is an “early-clobber” output. Marking an output as “early-clobber” ensures that LLVM will not use the same register for any inputs (other than an input tied to this output).
```
The eax is an output, so front-end need to mark the early-clobber for eax with "=&{eax}", so it seems to be a front-end issue?


Repository:
  rG LLVM Github Monorepo

CHANGES SINCE LAST ACTION
  https://reviews.llvm.org/D93983/new/

https://reviews.llvm.org/D93983