llvm.noalias - ObjectSizeOffsetVisitor should look through them

[hfinkel]

Summary

This is part of the series started by D9375, and teaches ObjectSizeOffsetVisitor to look through llvm.noalias when it looks through bitcasts (which is only does via an initial stripping for some reason).

[hfinkel]

And this is the wrong patch...

lib/Analysis/ValueTracking.cpp

 /// This function computes the length of a null-terminated C string pointed to
 /// by V. If successful, it returns true and returns the string in Str.
 /// If unsuccessful, it returns false.
 bool llvm::getConstantStringInfo(const Value *V, StringRef &Str,
                                  uint64_t Offset, bool TrimAtNul) {
   assert(V);
 
   // Look through bitcast instructions and geps.
-  V = V->stripPointerCasts();
+  V = V->stripPointerCasts(/*LookThroughNoAlias*/true);
 
   // If the value is a GEP instruction or constant expression, treat it as an
   // offset.
   if (const GEPOperator *GEP = dyn_cast<GEPOperator>(V)) {
     // Make sure the GEP has exactly three arguments.
     if (GEP->getNumOperands() != 3)
       return false;
 

 // These next two are very similar to the above, but also look through PHI
 // nodes.
 // TODO: See if we can integrate these two together.
 
 /// If we can compute the length of the string pointed to by
 /// the specified pointer, return 'len+1'.  If we can't, return 0.
 static uint64_t GetStringLengthH(Value *V, SmallPtrSetImpl<PHINode*> &PHIs) {
   // Look through noop bitcast instructions.
-  V = V->stripPointerCasts();
+  V = V->stripPointerCasts(/*LookThroughNoAlias*/true);
 
   // If this is a PHI node, there are two cases: either we have already seen it
   // or we haven't.
   if (PHINode *PN = dyn_cast<PHINode>(V)) {
     if (!PHIs.insert(PN).second)
       return ~0ULL;  // already in the set.
 
     // If it was new, see if all the input strings are the same length.

test/Transforms/InstCombine/strlen-1.ll

 define i32 @test_simplify1() {
 ; CHECK-LABEL: @test_simplify1(
   %hello_p = getelementptr [6 x i8], [6 x i8]* @hello, i32 0, i32 0
   %hello_l = call i32 @strlen(i8* %hello_p)
   ret i32 %hello_l
 ; CHECK-NEXT: ret i32 5
 }
 
+define i32 @test_simplify1a() {
+; CHECK-LABEL: @test_simplify1a(
+  %hello_p = getelementptr [6 x i8], [6 x i8]* @hello, i32 0, i32 0
+  %hello_pa = call i8* @llvm.noalias.p0i8(i8* %hello_p, metadata !1)
+  %hello_l = call i32 @strlen(i8* %hello_pa)
+  ret i32 %hello_l
+; CHECK-NEXT: ret i32 5
+}
+
 define i32 @test_simplify2() {
 ; CHECK-LABEL: @test_simplify2(
   %null_p = getelementptr [1 x i8], [1 x i8]* @null, i32 0, i32 0
   %null_l = call i32 @strlen(i8* %null_p)
   ret i32 %null_l
 ; CHECK-NEXT: ret i32 0
 }
 

 define i32 @test_no_simplify1() {
 ; CHECK-LABEL: @test_no_simplify1(
   %a_p = getelementptr [32 x i8], [32 x i8]* @a, i32 0, i32 0
   %a_l = call i32 @strlen(i8* %a_p)
 ; CHECK-NEXT: %a_l = call i32 @strlen
   ret i32 %a_l
 ; CHECK-NEXT: ret i32 %a_l
 }
+
+declare i8* @llvm.noalias.p0i8(i8*, metadata) nounwind
+
+!0 = !{!0, !"some domain"}
+!1 = !{!1, !0, !"some scope"}
+