[PATCH] D85886: [SimplifyLibCalls] Remove over-eager early return in strlen optzns.

[Florian Hahn via Phabricator via llvm-commits]

fhahn created this revision.
fhahn added reviewers: RKSimon, spatel, lebedev.ri.
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Currently we bail out early for strlen calls with a GEP operand, if none
of the GEP specific optimizations fire. But there could be later
optimizations that still apply,  which we currently miss out on.

An example is that we do not apply the following optimization

  strlen(x) == 0 --> *x == 0

Unless I am missing something, there seems to be no reason for bailing
out early there.

Fixes PR47149.


Repository:
  rG LLVM Github Monorepo

https://reviews.llvm.org/D85886

Files:
  llvm/lib/Transforms/Utils/SimplifyLibCalls.cpp
  llvm/test/Transforms/InstCombine/strlen-1.ll


Index: llvm/test/Transforms/InstCombine/strlen-1.ll
===================================================================
--- llvm/test/Transforms/InstCombine/strlen-1.ll
+++ llvm/test/Transforms/InstCombine/strlen-1.ll
@@ -223,9 +223,7 @@
 define i1 @strlen0_after_write_to_first_byte_global() {
 ; CHECK-LABEL: @strlen0_after_write_to_first_byte_global(
 ; CHECK-NEXT:    store i8 49, i8* getelementptr inbounds ([32 x i8], [32 x i8]* @a, i32 0, i32 0), align 16
-; CHECK-NEXT:    [[LEN:%.*]] = tail call i32 @strlen(i8* nonnull dereferenceable(1) getelementptr inbounds ([32 x i8], [32 x i8]* @a, i32 0, i32 0))
-; CHECK-NEXT:    [[CMP:%.*]] = icmp eq i32 [[LEN]], 0
-; CHECK-NEXT:    ret i1 [[CMP]]
+; CHECK-NEXT:    ret i1 false
 ;
   store i8 49, i8* getelementptr inbounds ([32 x i8], [32 x i8]* @a, i64 0, i64 0), align 16
   %len = tail call i32 @strlen(i8* nonnull dereferenceable(1) getelementptr inbounds ([32 x i8], [32 x i8]* @a, i64 0, i64 0))
@@ -236,8 +234,8 @@
 define i1 @strlen0_after_write_to_second_byte_global() {
 ; CHECK-LABEL: @strlen0_after_write_to_second_byte_global(
 ; CHECK-NEXT:    store i8 49, i8* getelementptr inbounds ([32 x i8], [32 x i8]* @a, i32 0, i32 1), align 16
-; CHECK-NEXT:    [[LEN:%.*]] = tail call i32 @strlen(i8* nonnull dereferenceable(1) getelementptr inbounds ([32 x i8], [32 x i8]* @a, i32 0, i32 0))
-; CHECK-NEXT:    [[CMP:%.*]] = icmp eq i32 [[LEN]], 0
+; CHECK-NEXT:    [[STRLENFIRST:%.*]] = load i8, i8* getelementptr inbounds ([32 x i8], [32 x i8]* @a, i32 0, i32 0), align 1
+; CHECK-NEXT:    [[CMP:%.*]] = icmp eq i8 [[STRLENFIRST]], 0
 ; CHECK-NEXT:    ret i1 [[CMP]]
 ;
   store i8 49, i8* getelementptr inbounds ([32 x i8], [32 x i8]* @a, i64 0, i64 1), align 16
Index: llvm/lib/Transforms/Utils/SimplifyLibCalls.cpp
===================================================================
--- llvm/lib/Transforms/Utils/SimplifyLibCalls.cpp
+++ llvm/lib/Transforms/Utils/SimplifyLibCalls.cpp
@@ -683,8 +683,6 @@
                            Offset);
       }
     }
-
-    return nullptr;
   }
 
   // strlen(x?"foo":"bars") --> x ? 3 : 4


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