[llvm] [AArch64] Combine store (trunc X to <3 x i8>) to sequence of ST1.b. (PR #78637)

[Eli Friedman via llvm-commits]

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@@ -154,17 +154,12 @@ define <3 x i32> @load_v3i32(ptr %src) {
 define void @store_trunc_from_64bits(ptr %src, ptr %dst) {
 ; CHECK-LABEL: store_trunc_from_64bits:
 ; CHECK:       ; %bb.0: ; %entry
-; CHECK-NEXT:    sub sp, sp, #16
-; CHECK-NEXT:    .cfi_def_cfa_offset 16
-; CHECK-NEXT:    ldr s0, [x0]
-; CHECK-NEXT:    ldrh w8, [x0, #4]
-; CHECK-NEXT:    mov.h v0[2], w8
-; CHECK-NEXT:    xtn.8b v0, v0
-; CHECK-NEXT:    str s0, [sp, #12]
-; CHECK-NEXT:    ldrh w9, [sp, #12]
-; CHECK-NEXT:    strb w8, [x1, #2]
-; CHECK-NEXT:    strh w9, [x1]
-; CHECK-NEXT:    add sp, sp, #16
+; CHECK-NEXT:    add x8, x0, #4
+; CHECK-NEXT:    ld1r.4h { v0 }, [x8]
+; CHECK-NEXT:    ldr w8, [x0]
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efriedma-quic wrote:

This isn't really doing a good job of demonstrating what you're trying to do here... maybe add a testcase with some arithmetic, so the store and the load can't be combined together?

Also, this looks like it's getting miscompiled; it's only storing two bytes.

https://github.com/llvm/llvm-project/pull/78637