[AlignFromAssume] Bailout w/non-constant alignments (pr51680)

This is a bailout for pr51680.  This pass appears to assume that the alignment operand to an align tag on an assume bundle is constant.  This doesn't appear to be required anywhere, and clang happily generates non-constant alignments for cases such as this case taken from the bug report:

// clang -cc1 -triple powerpc64-- -S -O1 opal_pci-min.c
extern int a[];
long *b;
long c;
void *d(long, int *, int, long, long, long) __attribute__((__alloc_align__(6)));
void e() {
  b = d(c, a, 0, 0, 5, c);
  b[0] = 0;
}

This was exposed by a SCEV change which allowed a non-constant alignment to reach further into the pass' code.  We could generalize the pass, but for now, let's fix the crash.

(cherry picked from commit 9b45fd9)

llvm/lib/Transforms/Scalar/AlignmentFromAssumptions.cpp

@@ -221,6 +221,10 @@ bool AlignmentFromAssumptionsPass::extractAlignmentInfo(CallInst *I,
   AAPtr = AAPtr->stripPointerCastsSameRepresentation();
   AlignSCEV = SE->getSCEV(AlignOB.Inputs[1].get());
   AlignSCEV = SE->getTruncateOrZeroExtend(AlignSCEV, Int64Ty);
+  if (!isa<SCEVConstant>(AlignSCEV))
+    // Added to suppress a crash because consumer doesn't expect non-constant
+    // alignments in the assume bundle.  TODO: Consider generalizing caller.
+    return false;
   if (AlignOB.Inputs.size() == 3)
     OffSCEV = SE->getSCEV(AlignOB.Inputs[2].get());
   else

llvm/test/Transforms/AlignmentFromAssumptions/simple.ll

@@ -252,6 +252,19 @@ entry:
 ; CHECK: ret i32 undef
 }
 
+
+; Variable alignments appear to be legal, don't crash
+define i32 @pr51680(i32* nocapture %a, i32 %align) nounwind uwtable readonly {
+entry:
+  tail call void @llvm.assume(i1 true) ["align"(i32* %a, i32 %align)]
+  %0 = load i32, i32* %a, align 4
+  ret i32 %0
+
+; CHECK-LABEL: @pr51680
+; CHECK: load i32, i32* {{[^,]+}}, align 4
+; CHECK: ret i32
+}
+
 declare void @llvm.assume(i1) nounwind
 
 declare void @llvm.memset.p0i8.i64(i8* nocapture, i8, i64, i1) nounwind