DianQK wrote: > Hello. I think that if you removed undef from the first instruction the > result would still be incorrect. With: > > ``` > $x8 = ORRXrs $xzr, $x0, 0, implicit $w0 > $w8 = ORRWrs $wzr, $w0, 0, implicit-def $x8 > ```
I'm also curious about it, but this transformation has been around for a long time, so I'm assuming it's correct. I think there exists some other pass solution to this problem here. > The top bits are not zero into the function? Yes. Please see the link https://llvm.godbolt.org/z/zv3vz8zcs. ```asm _call_from_main: sub sp, sp, #32 stp x29, x30, [sp, #16] add x29, sp, #16 mov w8, #1 str w8, [sp, #4] ;;; We save `0x00000001` on stack add x0, sp, #4 ;;; Save the address bl _device_create_texture ldp x29, x30, [sp, #16] add sp, sp, #32 ret _device_create_texture: sub sp, sp, #32 stp x29, x30, [sp, #16] add x29, sp, #16 ldr x8, [x0] ;;; Load 8-byte, get `0x6f60400000000001` str x8, [sp] ldr w9, [x0, #8] str w9, [sp, #8] ubfx x1, x8, #32, #8 mov x0, x8 bl __ZN12repro_11790213TextureFormat17required_features17h5e4d5de64322154bE .. ret __ZN12repro_11790213TextureFormat17required_features17h5e4d5de64322154bE: mov x8, x0 ;;; x8/x0 is `0x6f60400000000001` mov w0, #0 adrp x9, LJTI10_0@PAGE add x9, x9, LJTI10_0@PAGEOFF adr x10, LBB10_1 ldrb w11, [x9, x8] ;;; We expect x8 is `0x0000000000000001` ``` In this function, we can't guarantee that `x8/x0` is 0x1, we have to do a transformation `mov w8, w8`. So I think this delete subregister copy instruction is based on a convention similar to this one. We can delete `mov w8, w0` on this: ```asm mov x8, x0 mov w8, w0 mov w8, w8 ``` If I use `-O0`, I can get the result: ```asm __ZN12repro_11790213TextureFormat17required_features17h5e4d5de64322154bE: sub sp, sp, #16 str w1, [sp, #4] subs w8, w0, #0 mov w8, w8 ; This looks like a transformation that guarantees that x8 is `0x0000000000000001`. ``` After that I found the following code in `lowerCopy`. https://github.com/llvm/llvm-project/blob/0808be47b8fbf0307d0b6f2eb45ba9bfe1b3ae65/llvm/lib/Target/AArch64/AArch64InstrInfo.cpp#L4422-L4429 If we lower the code to `mov x8, x0` instead of `mov w8, w0`, this may be somewhat unsafe. So this code leaves an undef. My PR was answering this comment and undef. https://github.com/llvm/llvm-project/pull/74682 _______________________________________________ lldb-commits mailing list lldb-commits@lists.llvm.org https://lists.llvm.org/cgi-bin/mailman/listinfo/lldb-commits
