[llvm] [CGP]: Optimize mul.overflow. (PR #148343)

[Alexis Engelke via llvm-commits]

================
@@ -261,21 +261,33 @@ define i128 @u128_mul(i128 %x, i128 %y) {
 
 define { i128, i8 } @u128_checked_mul(i128 %x, i128 %y) {
 ; CHECK-LABEL: u128_checked_mul:
-; CHECK:       // %bb.0:
+; CHECK:       // %bb.0: // %overflow.entry
+; CHECK-NEXT:    orr x8, x1, x3
+; CHECK-NEXT:    cbz x8, .LBB17_2
+; CHECK-NEXT:  // %bb.1: // %overflow
 ; CHECK-NEXT:    mul x9, x3, x0
 ; CHECK-NEXT:    cmp x1, #0
 ; CHECK-NEXT:    ccmp x3, #0, #4, ne
-; CHECK-NEXT:    umulh x8, x1, x2
-; CHECK-NEXT:    umulh x10, x3, x0
+; CHECK-NEXT:    umulh x10, x1, x2
+; CHECK-NEXT:    umulh x8, x3, x0
 ; CHECK-NEXT:    madd x9, x1, x2, x9
-; CHECK-NEXT:    ccmp xzr, x8, #0, eq
-; CHECK-NEXT:    umulh x11, x0, x2
 ; CHECK-NEXT:    ccmp xzr, x10, #0, eq
+; CHECK-NEXT:    umulh x11, x0, x2
+; CHECK-NEXT:    ccmp xzr, x8, #0, eq
 ; CHECK-NEXT:    mul x0, x0, x2
 ; CHECK-NEXT:    cset w8, ne
 ; CHECK-NEXT:    adds x1, x11, x9
 ; CHECK-NEXT:    csinc w8, w8, wzr, lo
-; CHECK-NEXT:    eor w2, w8, #0x1
+; CHECK-NEXT:    b .LBB17_3
+; CHECK-NEXT:  .LBB17_2: // %overflow.no
+; CHECK-NEXT:    umulh x8, x0, x2
+; CHECK-NEXT:    madd x8, x0, x3, x8
----------------
aengelke wrote:

This shouldn't be here, x1 and x3 are known to be zero at this point. The bic below also shouldn't be executed on the fast path. The fast path really should only have:

```
umulh x1, x0, x2
mul x0, x0, x2
mov w2, wzr // or mov w2, 0
```

Likewise on many other occasions.

https://github.com/llvm/llvm-project/pull/148343