Don't use dropflag hints when the type is dropless

[jonas-schievink]

Apparently we allocate and maintain non-working dropflag hints since June... In anticipation of a working implementation of on-stack drop flag hints, let's not spend even more time on types that don't even need to be dropped.

fn main() {
    let (i,j,k,l) = (0,0,0,0);
}

used to translate to (unoptimized only, of course):

define internal void @_ZN4main20ha8deb085c47920d8eaaE() unnamed_addr #0 {
entry-block:
  %dropflag_hint_10 = alloca i8
  %dropflag_hint_11 = alloca i8
  %dropflag_hint_12 = alloca i8
  %dropflag_hint_13 = alloca i8
  %const = alloca { i32, i32, i32, i32 }
  %i = alloca i32
  %j = alloca i32
  %k = alloca i32
  %l = alloca i32
  store i8 61, i8* %dropflag_hint_10
  store i8 61, i8* %dropflag_hint_11
  store i8 61, i8* %dropflag_hint_12
  store i8 61, i8* %dropflag_hint_13
  %0 = bitcast { i32, i32, i32, i32 }* %const to i8*
  call void @llvm.memcpy.p0i8.p0i8.i64(i8* %0, i8* bitcast ({ i32, i32, i32, i32 }* @const2752 to i8*), i64 16, i32 4, i1 false)
  %1 = getelementptr inbounds { i32, i32, i32, i32 }, { i32, i32, i32, i32 }* %const, i32 0, i32 0
  %2 = load i32, i32* %1, align 4
  store i32 %2, i32* %i, align 4
  %3 = getelementptr inbounds { i32, i32, i32, i32 }, { i32, i32, i32, i32 }* %const, i32 0, i32 1
  %4 = load i32, i32* %3, align 4
  store i32 %4, i32* %j, align 4
  %5 = getelementptr inbounds { i32, i32, i32, i32 }, { i32, i32, i32, i32 }* %const, i32 0, i32 2
  %6 = load i32, i32* %5, align 4
  store i32 %6, i32* %k, align 4
  %7 = getelementptr inbounds { i32, i32, i32, i32 }, { i32, i32, i32, i32 }* %const, i32 0, i32 3
  %8 = load i32, i32* %7, align 4
  store i32 %8, i32* %l, align 4
  ret void
}

Now it gives:

define internal void @_ZN4main20ha8deb085c47920d8eaaE() unnamed_addr #0 {
entry-block:
  %const = alloca { i32, i32, i32, i32 }
  %i = alloca i32
  %j = alloca i32
  %k = alloca i32
  %l = alloca i32
  %0 = bitcast { i32, i32, i32, i32 }* %const to i8*
  call void @llvm.memcpy.p0i8.p0i8.i64(i8* %0, i8* bitcast ({ i32, i32, i32, i32 }* @const2748 to i8*), i64 16, i32 4, i1 false)
  %1 = getelementptr inbounds { i32, i32, i32, i32 }, { i32, i32, i32, i32 }* %const, i32 0, i32 0
  %2 = load i32, i32* %1, align 4
  store i32 %2, i32* %i, align 4
  %3 = getelementptr inbounds { i32, i32, i32, i32 }, { i32, i32, i32, i32 }* %const, i32 0, i32 1
  %4 = load i32, i32* %3, align 4
  store i32 %4, i32* %j, align 4
  %5 = getelementptr inbounds { i32, i32, i32, i32 }, { i32, i32, i32, i32 }* %const, i32 0, i32 2
  %6 = load i32, i32* %5, align 4
  store i32 %6, i32* %k, align 4
  %7 = getelementptr inbounds { i32, i32, i32, i32 }, { i32, i32, i32, i32 }* %const, i32 0, i32 3
  %8 = load i32, i32* %7, align 4
  store i32 %8, i32* %l, align 4
  ret void
}

Let's hope I didn't break anything!

[rust-highfive]

r? @alexcrichton

(rust_highfive has picked a reviewer for you, use r? to override)

[alexcrichton]

r? @pnkfelix

[pnkfelix]

Ironically this same bug was just pointed out to me at lunch today.

(I didn't have time to investigate as i have hands full with regression fixes at moment but will review this ASAP)

[pnkfelix]

@bors r+

[bors]

📌 Commit e3abc3c has been approved by pnkfelix