README.txt

  %67 = call i64 @llvm.objectsize.i64(i8* %64, i1 false) nounwind
  %68 = call i8* @__memset_chk(i8* %64, i32 0, i64 %62, i64 %67) nounwind

llvm.objectsize.i64 should be taught about malloc/calloc, allowing it to
fold to %62.  This is a security win (overflows of malloc will get caught)
and also a performance win by exposing more memsets to the optimizer.

This occurs several times in viterbi.

Note that this would change the semantics of @llvm.objectsize which by its
current definition always folds to a constant. We also should make sure that
we remove checking in code like

  char *p = malloc(strlen(s)+1);
  __strcpy_chk(p, s, __builtin_objectsize(p, 0));

//===---------------------------------------------------------------------===//

This code (from Benchmarks/Dhrystone/dry.c):

define i32 @Func1(i32, i32) nounwind readnone optsize ssp {
entry:
  %sext = shl i32 %0, 24
  %conv = ashr i32 %sext, 24
  %sext6 = shl i32 %1, 24
  %conv4 = ashr i32 %sext6, 24
  %cmp = icmp eq i32 %conv, %conv4
  %. = select i1 %cmp, i32 10000, i32 0
  ret i32 %.
}

Should be simplified into something like:

define i32 @Func1(i32, i32) nounwind readnone optsize ssp {
entry:
  %sext = shl i32 %0, 24
  %conv = and i32 %sext, 0xFF000000
  %sext6 = shl i32 %1, 24
  %conv4 = and i32 %sext6, 0xFF000000
  %cmp = icmp eq i32 %conv, %conv4
  %. = select i1 %cmp, i32 10000, i32 0
  ret i32 %.
}

and then to:

define i32 @Func1(i32, i32) nounwind readnone optsize ssp {
entry:
  %conv = and i32 %0, 0xFF
  %conv4 = and i32 %1, 0xFF
  %cmp = icmp eq i32 %conv, %conv4
  %. = select i1 %cmp, i32 10000, i32 0
  ret i32 %.
}
//===---------------------------------------------------------------------===//

clang -O3 currently compiles this code

int g(unsigned int a) {
  unsigned int c[100];
  c[10] = a;
  c[11] = a;
  unsigned int b = c[10] + c[11];
  if(b > a*2) a = 4;
  else a = 8;
  return a + 7;
}

into

define i32 @g(i32 a) nounwind readnone {
  %add = shl i32 %a, 1
  %mul = shl i32 %a, 1
  %cmp = icmp ugt i32 %add, %mul
  %a.addr.0 = select i1 %cmp, i32 11, i32 15
  ret i32 %a.addr.0
}

The icmp should fold to false. This CSE opportunity is only available
after GVN and InstCombine have run.

//===---------------------------------------------------------------------===//

memcpyopt should turn this:

define i8* @test10(i32 %x) {
  %alloc = call noalias i8* @malloc(i32 %x) nounwind
  call void @llvm.memset.p0i8.i32(i8* %alloc, i8 0, i32 %x, i32 1, i1 false)
  ret i8* %alloc
}

into a call to calloc.  We should make sure that we analyze calloc as
aggressively as malloc though.

//===---------------------------------------------------------------------===//

clang -O3 doesn't optimize this:

void f1(int* begin, int* end) {
  std::fill(begin, end, 0);
}

into a memset.  This is PR8942.

//===---------------------------------------------------------------------===//

clang -O3 -fno-exceptions currently compiles this code:

void f(int N) {
  std::vector<int> v(N);

  extern void sink(void*); sink(&v);
}

into

define void @_Z1fi(i32 %N) nounwind {
entry:
  %v2 = alloca [3 x i32*], align 8
  %v2.sub = getelementptr inbounds [3 x i32*]* %v2, i64 0, i64 0
  %tmpcast = bitcast [3 x i32*]* %v2 to %"class.std::vector"*
  %conv = sext i32 %N to i64
  store i32* null, i32** %v2.sub, align 8, !tbaa !0
  %tmp3.i.i.i.i.i = getelementptr inbounds [3 x i32*]* %v2, i64 0, i64 1
  store i32* null, i32** %tmp3.i.i.i.i.i, align 8, !tbaa !0
  %tmp4.i.i.i.i.i = getelementptr inbounds [3 x i32*]* %v2, i64 0, i64 2
  store i32* null, i32** %tmp4.i.i.i.i.i, align 8, !tbaa !0
  %cmp.i.i.i.i = icmp eq i32 %N, 0
  br i1 %cmp.i.i.i.i, label %_ZNSt12_Vector_baseIiSaIiEEC2EmRKS0_.exit.thread.i.i, label %cond.true.i.i.i.i

_ZNSt12_Vector_baseIiSaIiEEC2EmRKS0_.exit.thread.i.i: ; preds = %entry
  store i32* null, i32** %v2.sub, align 8, !tbaa !0
  store i32* null, i32** %tmp3.i.i.i.i.i, align 8, !tbaa !0
  %add.ptr.i5.i.i = getelementptr inbounds i32* null, i64 %conv
  store i32* %add.ptr.i5.i.i, i32** %tmp4.i.i.i.i.i, align 8, !tbaa !0
  br label %_ZNSt6vectorIiSaIiEEC1EmRKiRKS0_.exit

cond.true.i.i.i.i:                                ; preds = %entry
  %cmp.i.i.i.i.i = icmp slt i32 %N, 0
  br i1 %cmp.i.i.i.i.i, label %if.then.i.i.i.i.i, label %_ZNSt12_Vector_baseIiSaIiEEC2EmRKS0_.exit.i.i

if.then.i.i.i.i.i:                                ; preds = %cond.true.i.i.i.i
  call void @_ZSt17__throw_bad_allocv() noreturn nounwind
  unreachable

_ZNSt12_Vector_baseIiSaIiEEC2EmRKS0_.exit.i.i:    ; preds = %cond.true.i.i.i.i
  %mul.i.i.i.i.i = shl i64 %conv, 2
  %call3.i.i.i.i.i = call noalias i8* @_Znwm(i64 %mul.i.i.i.i.i) nounwind
  %0 = bitcast i8* %call3.i.i.i.i.i to i32*
  store i32* %0, i32** %v2.sub, align 8, !tbaa !0
  store i32* %0, i32** %tmp3.i.i.i.i.i, align 8, !tbaa !0
  %add.ptr.i.i.i = getelementptr inbounds i32* %0, i64 %conv
  store i32* %add.ptr.i.i.i, i32** %tmp4.i.i.i.i.i, align 8, !tbaa !0
  call void @llvm.memset.p0i8.i64(i8* %call3.i.i.i.i.i, i8 0, i64 %mul.i.i.i.i.i, i32 4, i1 false)
  br label %_ZNSt6vectorIiSaIiEEC1EmRKiRKS0_.exit

This is just the handling the construction of the vector. Most surprising here
is the fact that all three null stores in %entry are dead, but not eliminated.
Also surprising is that %conv isn't simplified to 0 in %....exit.thread.i.i.

//===---------------------------------------------------------------------===//

clang -O3 -fno-exceptions currently compiles this code:

void f(int N) {
  std::vector<int> v(N);
  for (int k = 0; k < N; ++k)
    v[k] = 0;

  extern void sink(void*); sink(&v);
}

into almost the same as the previous note, but replace its final BB with:

for.body.lr.ph:                                   ; preds = %cond.true.i.i.i.i
  %mul.i.i.i.i.i = shl i64 %conv, 2
  %call3.i.i.i.i.i = call noalias i8* @_Znwm(i64 %mul.i.i.i.i.i) nounwind
  %0 = bitcast i8* %call3.i.i.i.i.i to i32*
  store i32* %0, i32** %v8.sub, align 8, !tbaa !0
  %add.ptr.i.i.i = getelementptr inbounds i32* %0, i64 %conv
  store i32* %add.ptr.i.i.i, i32** %tmp4.i.i.i.i.i, align 8, !tbaa !0
  call void @llvm.memset.p0i8.i64(i8* %call3.i.i.i.i.i, i8 0, i64 %mul.i.i.i.i.i, i32 4, i1 false)
  store i32* %add.ptr.i.i.i, i32** %tmp3.i.i.i.i.i, align 8, !tbaa !0
  %tmp18 = add i32 %N, -1
  %tmp19 = zext i32 %tmp18 to i64
  %tmp20 = shl i64 %tmp19, 2
  %tmp21 = add i64 %tmp20, 4
  call void @llvm.memset.p0i8.i64(i8* %call3.i.i.i.i.i, i8 0, i64 %tmp21, i32 4, i1 false)
  br label %for.end

First off, why (((zext %N - 1) << 2) + 4) instead of the ((sext %N) << 2) done
previously? (or better yet, re-use that one?)

Then, the really painful one is the second memset, of the same memory, to the
same value.

//===---------------------------------------------------------------------===//

clang -O3 -fno-exceptions currently compiles this code:

struct S {
  unsigned short m1, m2;
  unsigned char m3, m4;
};

void f(int N) {
  std::vector<S> v(N);
  extern void sink(void*); sink(&v);
}

into poor code for zero-initializing 'v' when N is >0. The problem is that
S is only 6 bytes, but each element is 8 byte-aligned. We generate a loop and
4 stores on each iteration. If the struct were 8 bytes, this gets turned into
a memset.

//===---------------------------------------------------------------------===//

clang -O3 currently compiles this code:

extern const int magic;
double f() { return 0.0 * magic; }

into

@magic = external constant i32

define double @_Z1fv() nounwind readnone {
entry:
  %tmp = load i32* @magic, align 4, !tbaa !0
  %conv = sitofp i32 %tmp to double
  %mul = fmul double %conv, 0.000000e+00
  ret double %mul
}

We should be able to fold away this fmul to 0.0.  More generally, fmul(x,0.0)
can be folded to 0.0 if we can prove that the LHS is not -0.0, not a NaN, and
not an INF.  The CannotBeNegativeZero predicate in value tracking should be
extended to support general "fpclassify" operations that can return 
yes/no/unknown for each of these predicates.

In this predicate, we know that uitofp is trivially never NaN or -0.0, and
we know that it isn't +/-Inf if the floating point type has enough exponent bits
to represent the largest integer value as < inf.

//===---------------------------------------------------------------------===//

When optimizing a transformation that can change the sign of 0.0 (such as the
0.0*val -> 0.0 transformation above), it might be provable that the sign of the
expression doesn't matter.  For example, by the above rules, we can't transform
fmul(sitofp(x), 0.0) into 0.0, because x might be -1 and the result of the
expression is defined to be -0.0.

If we look at the uses of the fmul for example, we might be able to prove that
all uses don't care about the sign of zero.  For example, if we have:

  fadd(fmul(sitofp(x), 0.0), 2.0)

Since we know that x+2.0 doesn't care about the sign of any zeros in X, we can
transform the fmul to 0.0, and then the fadd to 2.0.

//===---------------------------------------------------------------------===//

We should enhance memcpy/memcpy/memset to allow a metadata node on them
indicating that some bytes of the transfer are undefined.  This is useful for
frontends like clang when lowering struct lowering, when some elements of the
struct are undefined.  Consider something like this:

struct x {
  char a;
  int b[4];
};
void foo(struct x*P);
struct x testfunc() {
  struct x V1, V2;
  foo(&V1);
  V2 = V1;

  return V2;
}

We currently compile this to:
$ clang t.c -S -o - -O0 -emit-llvm | opt -scalarrepl -S


%struct.x = type { i8, [4 x i32] }

define void @testfunc(%struct.x* sret %agg.result) nounwind ssp {
entry:
  %V1 = alloca %struct.x, align 4
  call void @foo(%struct.x* %V1)
  %tmp1 = bitcast %struct.x* %V1 to i8*
  %0 = bitcast %struct.x* %V1 to i160*
  %srcval1 = load i160* %0, align 4
  %tmp2 = bitcast %struct.x* %agg.result to i8*
  %1 = bitcast %struct.x* %agg.result to i160*
  store i160 %srcval1, i160* %1, align 4
  ret void
}

This happens because SRoA sees that the temp alloca has is being memcpy'd into
and out of and it has holes and it has to be conservative.  If we knew about the
holes, then this could be much much better.

Having information about these holes would also improve memcpy (etc) lowering at
llc time when it gets inlined, because we can use smaller transfers.  This also
avoids partial register stalls in some important cases.

//===---------------------------------------------------------------------===//