llvm dev - Mar 2015 - [LLVMdev] Inline Assembly: Memory constraints with offsets

Hi,

I'm trying to implement the ZC inline assembly constraint for Mips. This
constraint is a memory constraint that expands to an address with an offset (the
range of the offset varies according to the subtarget), so the inline assembly
in:
  int data[10];
  void ZC(void) {
    asm volatile ("foo %0 %1" : : "ZC"(data[1]),
"ZC"(data[2]));
  }
Should expand to something like:
  foo 4($2) 8($2)

At the moment, the best I can get is something like:
  foo 0($2) 0($3)
with the offsets being added before the inline assembly.

Does anyone have any suggestions as to how I can get the offset inside the
inline assembly?

Thanks

Daniel Sanders
Leading Software Design Engineer, MIPS Processor IP
Imagination Technologies Limited
www.imgtec.com<http://www.imgtec.com/>

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On 3/3/2015 6:01 AM, Daniel Sanders wrote:
> Hi,
>
> I'm trying to implement the ZC inline assembly constraint for Mips.
This
> constraint is a memory constraint that expands to an address with an
> offset (the range of the offset varies according to the subtarget), so
> the inline assembly in:
>
>    int data[10];
>
>    void ZC(void) {
>
>      asm volatile ("foo %0 %1" : : "ZC"(data[1]),
"ZC"(data[2]));
>
>    }
>
> Should expand to something like:
>
>    foo 4($2) 8($2)
>
> At the moment, the best I can get is something like:
>
>    foo 0($2) 0($3)
>
> with the offsets being added before the inline assembly.
>
> Does anyone have any suggestions as to how I can get the offset inside
> the inline assembly?
How are you actually getting this to compile?  I just built clang and 
I'm getting an error:

clang-3.6 -target mips -S mipsa.c
mipsa.c:4:33: error: invalid input constraint 'ZC' in asm
   asm volatile ("foo %0 %1" : : "ZC"(data[1]),
"ZC"(data[2]));
                                 ^
1 error generated.


It doesn't seem that the function "validateAsmConstraint" in 
tools/clang/lib/Basic/Targets.cpp handles "ZC" as a constraint.

-Krzysztof


-- 
Qualcomm Innovation Center, Inc. is a member of Code Aurora Forum, 
hosted by The Linux Foundation

> Does anyone have any suggestions as to how I can get the offset inside the
> inline assembly?
I think x86 has the same capability with a simple "m" constraint.
Looks like it's based on the "SelectInlineAsmMemoryOperand"
function.

Cheers.

Tim.

> -----Original Message-----
> From: Tim Northover [mailto:t.p.northover at gmail.com]
> Sent: 03 March 2015 14:42
> To: Daniel Sanders
> Cc: LLVM Developers Mailing List (llvmdev at cs.uiuc.edu)
> Subject: Re: [LLVMdev] Inline Assembly: Memory constraints with offsets
> 
> > Does anyone have any suggestions as to how I can get the offset inside
the
> > inline assembly?
> 
> I think x86 has the same capability with a simple "m" constraint.
> Looks like it's based on the "SelectInlineAsmMemoryOperand"
function.
> 
> Cheers.
> 
> Tim.
Thanks. I came across SelectInlineAsmMemoryOperand but the caller I found is
passing a hardcoded 'm' into it. I'll start by seeing if the
'o' and 'v' paths really trigger for X86.

> -----Original Message-----
> From: llvmdev-bounces at cs.uiuc.edu [mailto:llvmdev-bounces at
cs.uiuc.edu]
> On Behalf Of Krzysztof Parzyszek
> Sent: 03 March 2015 14:35
> To: llvmdev at cs.uiuc.edu
> Subject: Re: [LLVMdev] Inline Assembly: Memory constraints with offsets
> 
> On 3/3/2015 6:01 AM, Daniel Sanders wrote:
> > Hi,
> >
> > I'm trying to implement the ZC inline assembly constraint for
Mips. This
> > constraint is a memory constraint that expands to an address with an
> > offset (the range of the offset varies according to the subtarget), so
> > the inline assembly in:
> >
> >    int data[10];
> >
> >    void ZC(void) {
> >
> >      asm volatile ("foo %0 %1" : : "ZC"(data[1]),
"ZC"(data[2]));
> >
> >    }
> >
> > Should expand to something like:
> >
> >    foo 4($2) 8($2)
> >
> > At the moment, the best I can get is something like:
> >
> >    foo 0($2) 0($3)
> >
> > with the offsets being added before the inline assembly.
> >
> > Does anyone have any suggestions as to how I can get the offset inside
> > the inline assembly?
> 
> How are you actually getting this to compile?  I just built clang and
> I'm getting an error:
> 
> clang-3.6 -target mips -S mipsa.c
> mipsa.c:4:33: error: invalid input constraint 'ZC' in asm
>    asm volatile ("foo %0 %1" : : "ZC"(data[1]),
"ZC"(data[2]));
>                                  ^
> 1 error generated.
> 
> 
> It doesn't seem that the function "validateAsmConstraint" in
> tools/clang/lib/Basic/Targets.cpp handles "ZC" as a constraint.
> 
> -Krzysztof
Partial support for ZC is in my working copy at the moment. I've attached my
WIP patches.
 
> --
> Qualcomm Innovation Center, Inc. is a member of Code Aurora Forum,
> hosted by The Linux Foundation
> _______________________________________________
> LLVM Developers mailing list
> LLVMdev at cs.uiuc.edu         http://llvm.cs.uiuc.edu
> http://lists.cs.uiuc.edu/mailman/listinfo/llvmdev
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> -----Original Message-----
> From: Daniel Sanders
> Sent: 03 March 2015 16:23
> To: 'Tim Northover'
> Cc: LLVM Developers Mailing List (llvmdev at cs.uiuc.edu)
> Subject: RE: [LLVMdev] Inline Assembly: Memory constraints with offsets
> 
> > -----Original Message-----
> > From: Tim Northover [mailto:t.p.northover at gmail.com]
> > Sent: 03 March 2015 14:42
> > To: Daniel Sanders
> > Cc: LLVM Developers Mailing List (llvmdev at cs.uiuc.edu)
> > Subject: Re: [LLVMdev] Inline Assembly: Memory constraints with
offsets
> >
> > > Does anyone have any suggestions as to how I can get the offset
inside
> the
> > > inline assembly?
> >
> > I think x86 has the same capability with a simple "m"
constraint.
> > Looks like it's based on the
"SelectInlineAsmMemoryOperand" function.
> >
> > Cheers.
> >
> > Tim.
> 
> Thanks. I came across SelectInlineAsmMemoryOperand but the caller I found
> is passing a hardcoded 'm' into it. I'll start by seeing if the
'o' and 'v' paths
> really trigger for X86.
It turns out that those paths don't trigger. I've made a small change to
print the ConstraintCode when this function is called and found that the
following code:
  int data[10];
  void o(void) {
    asm volatile ("foo %0" : : "o"(data[1]));
  }
  // The 'v' case is commented out because clang doesn't actually
accept it in the frontend despite having a backend implementation.
  //void v(void) {
  //  asm volatile ("foo %0" : : "v"(data[1]));
  //}
  void m(void) {
    asm volatile ("foo %0" : : "m"(data[1]));
  }
when compiled for X86, only prints 'm' during compilation and never
'o'. It appears that only 'm' and equivalent constraints can be
implemented.

I think the 'm' is hardcoded because the constraints aren't present
in the SelectionDAG nodes and I think they're not present because it's
difficult to represent them. I expect the 'm' was good enough at the
time and Mips might be the first target to need sufficiently complicated
constraints for it to be a problem. I'll have to have a think as to how I
can deliver the constraint code to this function nicely.