[llvm] [SROA] analyze gep(ptr,phi(const...)) (PR #82425)

[Artem Belevich via llvm-commits]

================
@@ -504,6 +505,47 @@ bb.4:                                                ; preds = %bb.1, %bb.1, %bb
   ret i32 %load
 }
 
+
+define void @test_gep_phi_const(i32 %arg) {
+; CHECK-LABEL: @test_gep_phi_const(
+; CHECK-NEXT:  bb:
+; CHECK-NEXT:    switch i32 [[ARG:%.*]], label [[BB9:%.*]] [
+; CHECK-NEXT:      i32 0, label [[BB11:%.*]]
+; CHECK-NEXT:      i32 1, label [[BB10:%.*]]
+; CHECK-NEXT:    ]
+; CHECK:       bb9:
+; CHECK-NEXT:    [[PHI:%.*]] = phi i64 [ 8, [[BB10]] ], [ 16, [[BB:%.*]] ]
+; CHECK-NEXT:    [[PHI_SROA_PHI_SROA_SPECULATED:%.*]] = phi ptr [ undef, [[BB10]] ], [ undef, [[BB]] ]
+; CHECK-NEXT:    br label [[BB11]]
+; CHECK:       bb10:
+; CHECK-NEXT:    br label [[BB9]]
+; CHECK:       bb11:
+; CHECK-NEXT:    [[PHI12:%.*]] = phi ptr [ [[PHI_SROA_PHI_SROA_SPECULATED]], [[BB9]] ], [ undef, [[BB]] ]
+; CHECK-NEXT:    store i8 0, ptr [[PHI12]], align 1
+; CHECK-NEXT:    ret void
+;
+bb:
+  %alloca = alloca %t1, align 8
+  switch i32 %arg, label %bb9 [
+  i32 0, label %bb11
+  i32 1, label %bb10
+  ]
+
+bb9:                                              ; preds = %bb10, %bb
+  %phi = phi i64 [ 8, %bb10 ], [ 16, %bb ]
+  %getelementptr = getelementptr i8, ptr %alloca, i64 %phi
+  %load = load ptr, ptr %getelementptr, align 8
+  br label %bb11
+
+bb10:                                             ; preds = %bb
+  br label %bb9
+
+bb11:                                             ; preds = %bb9, %bb
+  %phi12 = phi ptr [ %load, %bb9 ], [ undef, %bb ]
+  store i8 0, ptr %phi12, align 1
+  ret void
+}
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Artem-B wrote:

Would the test below do? It verifies that we only handle all-const phi indices. Or did you have something else in mind?

That got me thinking. Perhaps we should relax "phi(all-const)" into `phi(any-const)` as it will give SROA some more slices to analyze.

https://github.com/llvm/llvm-project/pull/82425