[PATCH] D124836: [AArch64] Add support for -fzero-call-used-regs

[Bill Wendling via Phabricator via llvm-commits]

void added inline comments.


================
Comment at: llvm/lib/Target/AArch64/AArch64FrameLowering.cpp:682
+    // The called routine is expected to preserve r19-r28
+    // r29 and r30 are used as frame pointer and link register resp.
+    return 0;
----------------
nickdesaulniers wrote:
> What happens if `-fomit-frame-pointer` is specified? Is X29 used as a GPR then?
GCC only clears registers R0-R18 with or without `-fomit-frame-pointer`. (That's using `-fzero-call-used-regs=all`, so register usage isn't a consideration.) I assume that it's correct, or at least close to it.


================
Comment at: llvm/lib/Target/AArch64/AArch64FrameLowering.cpp:752
+#undef CASE
+  }
+}
----------------
peterwaller-arm wrote:
> void wrote:
> > kristof.beyls wrote:
> > > Just a drive-by comment: I'm wondering if SVE registers should also be listed here?
> > I'm not familiar with the SVE registers (I assume you mean the `Z#` and `P#` ones). Could you give an example program?
> SVE is slightly tricker here because the set of registers the caller must preserve depends on the signature of the function.
> 
> This is described here: https://github.com/ARM-software/abi-aa/blob/8a7b266879c60ca1c76e94ebb279b2dac60ed6a5/aapcs64/aapcs64.rst#613scalable-vector-registers
> 
> The callee-preserved registers are z8-z23 and p4-p15 if the function is using the VARIANT_PCS, the code for that condition in the asm printer is here:
> 
> https://github.com/llvm/llvm-project/blob/78fd413cf736953ac623cabf3d5f84c8219e31f8/llvm/lib/Target/AArch64/AArch64AsmPrinter.cpp#L864-L875
> 
> 
> ```
>  if (MF->getFunction().getCallingConv() == CallingConv::AArch64_VectorCall ||
>       MF->getFunction().getCallingConv() ==
>           CallingConv::AArch64_SVE_VectorCall ||
>       STI->getRegisterInfo()->hasSVEArgsOrReturn(MF)) {
> ```
> 
> Hope that helps a little.
Okay, so GCC does clear out SVE registers when using `-march=armv8-a+sve`:

```
        mov     z0.h, #0
        mov     z1.h, #0
        mov     z2.h, #0
        mov     z3.h, #0
        mov     z4.h, #0
        mov     z5.h, #0
        mov     z6.h, #0
        mov     z7.h, #0
        mov     z16.h, #0
        mov     z17.h, #0
        mov     z18.h, #0
        mov     z19.h, #0
        mov     z20.h, #0
        mov     z21.h, #0
        mov     z22.h, #0
        mov     z23.h, #0
        mov     z24.h, #0
        mov     z25.h, #0
        mov     z26.h, #0
        mov     z27.h, #0
        mov     z28.h, #0
        mov     z29.h, #0
        mov     z30.h, #0
        mov     z31.h, #0
        pfalse  p0.b
        pfalse  p1.b
        pfalse  p2.b
        pfalse  p3.b
        pfalse  p4.b
        pfalse  p5.b
        pfalse  p6.b
        pfalse  p7.b
        pfalse  p8.b
        pfalse  p9.b
        pfalse  p10.b
        pfalse  p11.b
        pfalse  p12.b
        pfalse  p13.b
        pfalse  p14.b
        pfalse  p15.b
```


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https://reviews.llvm.org/D124836