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Volatile loads and stores aren't honored #166

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yvt opened this issue Apr 25, 2022 · 1 comment · May be fixed by #167
Open

Volatile loads and stores aren't honored #166

yvt opened this issue Apr 25, 2022 · 1 comment · May be fixed by #167
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@yvt
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yvt commented Apr 25, 2022 

#[no_mangle]
pub unsafe fn assign_answer(x: *mut u32) {
    x.read_volatile();
    x.write_volatile(42);
    x.write_volatile(42);
}

In the code generated by cg_llvm, all volatile loads and stores are preserved.

0000000000001100 <assign_answer>:
    1100:       8b 07                   mov    (%rdi),%eax
    1102:       c7 07 2a 00 00 00       movl   $0x2a,(%rdi)
    1108:       c7 07 2a 00 00 00       movl   $0x2a,(%rdi)
    110e:       c3                      ret    
    110f:       90                      nop

In the code generated by cg_gcc, the load whose result is discarded is eliminated, and the overlapping stores are combined into one.

0000000000001100 <assign_answer>:
    1100:       c7 07 2a 00 00 00       movl   $0x2a,(%rdi)
    1106:       c3                      ret    
    1107:       66 0f 1f 84 00 00 00    nopw   0x0(%rax,%rax,1)
    110e:       00 00
@yvt
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yvt commented Apr 25, 2022 •
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Looks like the volatile qualification is incorrectly applied to the pointer type instead of its pointee.

rustc_codegen_gcc/src/builder.rs

Lines 671 to 675 in 1d62e95

fn volatile_load(&mut self, _ty: Type<'gcc>, ptr: RValue<'gcc>) -> RValue<'gcc> {
// TODO(antoyo): use ty.
let ptr = self.context.new_cast(None, ptr, ptr.get_type().make_volatile());
ptr.dereference(None).to_rvalue()
}

@antoyo antoyo added the bug Something isn't working label Apr 25, 2022
@yvt yvt linked a pull request Apr 26, 2022 that will close this issue
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@YakoYakoYokuYoku YakoYakoYokuYoku mentioned this issue Dec 24, 2023
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Implement volatile loads and stores yvt/rustc_codegen_gcc
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@antoyo @yvt