[PATCH] D75814: [InstCombine] Don't simplify calls without uses

[Nikita Popov via Phabricator via llvm-commits]

nikic created this revision.
nikic added reviewers: spatel, lebedev.ri, xbolva00.
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When simplifying a call without uses, replaceInstUsesWith() is going to do nothing, but we'll skip all following folds. We can only run into this problem with calls that both simplify and are not trivially dead if unused, which currently seems to happen only with calls to undef, as the test diff shows. When extending SimplifyCall() to handle "returned" attributes, this becomes a much bigger problem, so I'm fixing this first.


Repository:
  rG LLVM Github Monorepo

https://reviews.llvm.org/D75814

Files:
  lib/Transforms/InstCombine/InstCombineCalls.cpp
  test/Transforms/InstCombine/pr44245.ll


Index: test/Transforms/InstCombine/pr44245.ll
===================================================================
--- test/Transforms/InstCombine/pr44245.ll
+++ test/Transforms/InstCombine/pr44245.ll
@@ -59,7 +59,7 @@
 ; CHECK-NEXT:    br label [[BB47]]
 ; CHECK:       bb152:
 ; CHECK-NEXT:    [[TMP1848]] = load i8*, i8** inttoptr (i64 16 to i8**), align 16
-; CHECK-NEXT:    call void undef()
+; CHECK-NEXT:    store i1 true, i1* undef, align 1
 ; CHECK-NEXT:    br label [[BB150]]
 ;
 bb16:                                             ; preds = %bb
Index: lib/Transforms/InstCombine/InstCombineCalls.cpp
===================================================================
--- lib/Transforms/InstCombine/InstCombineCalls.cpp
+++ lib/Transforms/InstCombine/InstCombineCalls.cpp
@@ -1787,8 +1787,11 @@
 /// instructions. For normal calls, it allows visitCallBase to do the heavy
 /// lifting.
 Instruction *InstCombiner::visitCallInst(CallInst &CI) {
-  if (Value *V = SimplifyCall(&CI, SQ.getWithInstruction(&CI)))
-    return replaceInstUsesWith(CI, V);
+  // Don't try to simplify calls without uses. It will not do anything useful,
+  // but will result in the following folds being skipped.
+  if (!CI.use_empty())
+    if (Value *V = SimplifyCall(&CI, SQ.getWithInstruction(&CI)))
+      return replaceInstUsesWith(CI, V);
 
   if (isFreeCall(&CI, &TLI))
     return visitFree(CI);


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