[llvm] [PowerPC] Implement a more efficient memcmp in cases where the length is known. (PR #158657)

[via llvm-commits]

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@@ -122,23 +117,17 @@ define signext i32 @equalityFoldOneConstant(ptr %X) {
 ; CHECK-LABEL: equalityFoldOneConstant:
 ; CHECK:       # %bb.0:
 ; CHECK-NEXT:    li 5, 1
-; CHECK-NEXT:    ld 4, 0(3)
+; CHECK-NEXT:    ld 4, 8(3)
+; CHECK-NEXT:    ld 3, 0(3)
 ; CHECK-NEXT:    rldic 5, 5, 32, 31
-; CHECK-NEXT:    cmpld 4, 5
-; CHECK-NEXT:    bne 0, .LBB6_2
-; CHECK-NEXT:  # %bb.1: # %loadbb1
+; CHECK-NEXT:    xor 3, 3, 5
 ; CHECK-NEXT:    lis 5, -32768
-; CHECK-NEXT:    ld 4, 8(3)
-; CHECK-NEXT:    li 3, 0
 ; CHECK-NEXT:    ori 5, 5, 1
 ; CHECK-NEXT:    rldic 5, 5, 1, 30
-; CHECK-NEXT:    cmpld 4, 5
-; CHECK-NEXT:    beq 0, .LBB6_3
-; CHECK-NEXT:  .LBB6_2: # %res_block
-; CHECK-NEXT:    li 3, 1
-; CHECK-NEXT:  .LBB6_3: # %endblock
-; CHECK-NEXT:    cntlzw 3, 3
-; CHECK-NEXT:    srwi 3, 3, 5
+; CHECK-NEXT:    xor 4, 4, 5
+; CHECK-NEXT:    or 3, 3, 4
+; CHECK-NEXT:    cntlzd 3, 3
+; CHECK-NEXT:    rldicl 3, 3, 58, 63
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RolandF77 wrote:

Can we not change this sequence? It seems like a side effect and I'm not sure it's faster or slower.

https://github.com/llvm/llvm-project/pull/158657