llvm.noalias - computeKnownBits should look through them

[hfinkel]

Summary

This is part of the series started by D9375, and teaches computeKnownBits to look through calls to llvm.noalias. This is necessary for pointer-alignment propagation to work.

[reames]

LGTM

[hfinkel]

Rebased.

[majnemer]

lib/Analysis/ValueTracking.cpp

1463–1465

I wonder if it'd make sense to make this intrinsic's pointer parameter as returned and then teach computeKnownBitsFromOperator how to recurse onto a returned parameter.

[hfinkel]

lib/Analysis/ValueTracking.cpp

1463–1465

Excellent! This applies to many of the patches, however. I'll rework the relevant patches to be about the returned attribute, not the noalias intrinsic specifically. That makes me happier.

lib/Analysis/ValueTracking.cpp

case Instruction::Invoke:
     if (MDNode *MD = cast<Instruction>(I)->getMetadata(LLVMContext::MD_range))
       computeKnownBitsFromRangeMetadata(*MD, KnownZero);
     // If a range metadata is attached to this IntrinsicInst, intersect the
     // explicit range specified by the metadata and the implicit range of
     // the intrinsic.
     if (IntrinsicInst *II = dyn_cast<IntrinsicInst>(I)) {
       switch (II->getIntrinsicID()) {
       default: break;
+      case Intrinsic::noalias:
+        computeKnownBits(I->getOperand(0), KnownZero, KnownOne, DL, Depth + 1, Q);
+        break;
       case Intrinsic::ctlz:
       case Intrinsic::cttz: {
         unsigned LowBits = Log2_32(BitWidth)+1;
         // If this call is undefined for 0, the result will be less than 2^n.
         if (II->getArgOperand(1) == ConstantInt::getTrue(II->getContext()))
           LowBits -= 1;
         KnownZero |= APInt::getHighBitsSet(BitWidth, BitWidth - LowBits);
         break;

test/Transforms/InstCombine/align-attr.ll

 ; RUN: opt < %s -instcombine -S | FileCheck %s
 target datalayout = "e-m:e-i64:64-f80:128-n8:16:32:64-S128"
 target triple = "x86_64-unknown-linux-gnu"
 
 ; Function Attrs: nounwind uwtable
 define i32 @foo1(i32* align 32 %a) #0 {
 entry:
   %0 = load i32, i32* %a, align 4
   ret i32 %0
 
 ; CHECK-LABEL: @foo1
 ; CHECK-DAG: load i32, i32* %a, align 32
 ; CHECK: ret i32
 }
 
+define i32 @foo2(i32* align 32 %a) #0 {
+entry:
+  %v = call i32* @llvm.noalias.p0i32(i32* %a, metadata !1)
+  %0 = load i32, i32* %v, align 4
+  ret i32 %0
+
+; CHECK-LABEL: @foo2
+; CHECK-DAG: load i32, i32* %v, align 32
+; CHECK: ret i32
+}
+
+declare i32* @llvm.noalias.p0i32(i32*, metadata) nounwind
+
+!0 = !{!0, !"some domain"}
+!1 = !{!1, !0, !"some scope"}
+