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

1291–1293 ↗

(On Diff #63344)

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

1291–1293 ↗

(On Diff #63344)

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.

llvm/trunk/lib/Analysis/ValueTracking.cpp

if (Depth < MaxDepth - 1 && !KnownZero && !KnownOne) {
         if (!KnownZero && !KnownOne)
           break;
       }
     }
     break;
   }
   case Instruction::Call:
   case Instruction::Invoke:
+    // If range metadata is attached to this call, set known bits from that,
+    // and then intersect with known bits based on other properties of the
+    // function.
     if (MDNode *MD = cast<Instruction>(I)->getMetadata(LLVMContext::MD_range))
       computeKnownBitsFromRangeMetadata(*MD, KnownZero, KnownOne);
-    // 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 (Value *RV = CallSite(I).getReturnedArgOperand()) {
+      computeKnownBits(RV, KnownZero2, KnownOne2, Depth + 1, Q);
+      KnownZero |= KnownZero2;
+      KnownOne |= KnownOne2;
+    }
     if (IntrinsicInst *II = dyn_cast<IntrinsicInst>(I)) {
       switch (II->getIntrinsicID()) {
       default: break;
       case Intrinsic::bswap:
         computeKnownBits(I->getOperand(0), KnownZero2, KnownOne2, Depth + 1, Q);
         KnownZero |= KnownZero2.byteSwap();
         KnownOne |= KnownOne2.byteSwap();
         break;

llvm/trunk/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* @func1(i32* %a)
+  %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* @func1(i32* returned) nounwind
+