[llvm] [AArch64][SelectionDAG] Lower multiplication by a constant to shl+sub+shl+sub (PR #90199)

[via llvm-commits]

================
@@ -615,6 +632,42 @@ define i32 @test97_fast_shift(i32 %x) "target-features"="+alu-lsl-fast" {
   ret i32 %mul
 }
 
+; Negative test: The shift number 5 is out of bound
+define i32 @test125_fast_shift(i32 %x) "target-features"="+alu-lsl-fast" {
+; CHECK-LABEL: test125_fast_shift:
+; CHECK:       // %bb.0:
+; CHECK-NEXT:    mov w8, #125 // =0x7d
+; CHECK-NEXT:    mul w0, w0, w8
+; CHECK-NEXT:    ret
+;
+; GISEL-LABEL: test125_fast_shift:
+; GISEL:       // %bb.0:
+; GISEL-NEXT:    mov w8, #125 // =0x7d
+; GISEL-NEXT:    mul w0, w0, w8
+; GISEL-NEXT:    ret
+
+  %mul = mul nsw i32 %x, 125 ; 63 = 1 - ((1-32) << 2)
+  ret i32 %mul
+}
+
+; Negative test: The shift number 5 is out of bound
+define i32 @test225_fast_shift(i32 %x) "target-features"="+alu-lsl-fast" {
+; CHECK-LABEL: test225_fast_shift:
+; CHECK:       // %bb.0:
+; CHECK-NEXT:    mov w8, #225 // =0xe1
----------------
vfdff wrote:

oh, Yes.  225 = (1-16) * (1-16), which is not supported  now.


https://github.com/llvm/llvm-project/pull/90199