[llvm] [InstCombine] Relax the same-underlying-object constraint for the GEP canonicalization (PR #76583)

[Nikita Popov via llvm-commits]

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@@ -1537,4 +1537,82 @@ define ptr @gep_ashr_without_exact(ptr %p, i64 %off) {
   ret ptr %ptr
 }
 
+define i1 @test_only_used_by_icmp(ptr %a, ptr %b, ptr %c) {
+; CHECK-LABEL: @test_only_used_by_icmp(
+; CHECK-NEXT:    [[CMP:%.*]] = icmp eq ptr [[B:%.*]], [[C:%.*]]
+; CHECK-NEXT:    ret i1 [[CMP]]
+;
+  %pa = ptrtoint ptr %a to i64
+  %pb = ptrtoint ptr %b to i64
+  %sub = sub i64 %pb, %pa
+  %gep = getelementptr i8, ptr %a, i64 %sub
+  %cmp = icmp eq ptr %gep, %c
+  ret i1 %cmp
+}
+
+define i64 @test_only_used_by_ptrtoint(ptr %a, ptr %b) {
+; CHECK-LABEL: @test_only_used_by_ptrtoint(
+; CHECK-NEXT:    [[VAL:%.*]] = ptrtoint ptr [[B:%.*]] to i64
+; CHECK-NEXT:    ret i64 [[VAL]]
+;
+  %pa = ptrtoint ptr %a to i64
+  %pb = ptrtoint ptr %b to i64
+  %sub = sub i64 %pb, %pa
+  %gep = getelementptr i8, ptr %a, i64 %sub
+  %val = ptrtoint ptr %gep to i64
+  ret i64 %val
+}
+
+define i64 @test_used_by_both(ptr %a, ptr %b, ptr %c) {
+; CHECK-LABEL: @test_used_by_both(
+; CHECK-NEXT:    [[CMP:%.*]] = icmp eq ptr [[B:%.*]], [[C:%.*]]
+; CHECK-NEXT:    br i1 [[CMP]], label [[IF_THEN:%.*]], label [[IF_ELSE:%.*]]
+; CHECK:       if.then:
+; CHECK-NEXT:    [[VAL:%.*]] = ptrtoint ptr [[B]] to i64
+; CHECK-NEXT:    ret i64 [[VAL]]
+; CHECK:       if.else:
+; CHECK-NEXT:    ret i64 0
+;
+  %pa = ptrtoint ptr %a to i64
+  %pb = ptrtoint ptr %b to i64
+  %sub = sub i64 %pb, %pa
+  %gep = getelementptr i8, ptr %a, i64 %sub
+  %cmp = icmp eq ptr %gep, %c
+  br i1 %cmp, label %if.then, label %if.else
+if.then:
+  %val = ptrtoint ptr %gep to i64
+  ret i64 %val
+if.else:
+  ret i64 0
+}
+
+; Negative tests
+
+define i64 @test_used_by_both_invalid(ptr %a, ptr %b, ptr %c) {
+; CHECK-LABEL: @test_used_by_both_invalid(
+; CHECK-NEXT:    [[PA:%.*]] = ptrtoint ptr [[A:%.*]] to i64
+; CHECK-NEXT:    [[PB:%.*]] = ptrtoint ptr [[B:%.*]] to i64
+; CHECK-NEXT:    [[SUB:%.*]] = sub i64 [[PB]], [[PA]]
+; CHECK-NEXT:    [[GEP:%.*]] = getelementptr i8, ptr [[A]], i64 [[SUB]]
+; CHECK-NEXT:    [[CMP:%.*]] = icmp eq ptr [[GEP]], [[C:%.*]]
+; CHECK-NEXT:    br i1 [[CMP]], label [[IF_THEN:%.*]], label [[IF_ELSE:%.*]]
+; CHECK:       if.then:
+; CHECK-NEXT:    [[VAL:%.*]] = load i64, ptr [[GEP]], align 8
+; CHECK-NEXT:    ret i64 [[VAL]]
+; CHECK:       if.else:
+; CHECK-NEXT:    ret i64 0
+;
+  %pa = ptrtoint ptr %a to i64
+  %pb = ptrtoint ptr %b to i64
+  %sub = sub i64 %pb, %pa
+  %gep = getelementptr i8, ptr %a, i64 %sub
+  %cmp = icmp eq ptr %gep, %c
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nikic wrote:

We could still replace the use in the icmp here, in which case the rest would be sunk into the if.then block, which seems like a win.

This makes me wonder whether it would be better to solve this by adding extra transforms rooted at icmp and ptrtoint, so we can fold the uses in those instructions, while keeping the uses in other instructions?

https://github.com/llvm/llvm-project/pull/76583