Missed optimization?

[David Nadlinger via digitalmars-d-ldc]

On 21 Oct 2016, at 22:14, safety0ff via digitalmars-d-ldc wrote:
> I was experimenting with pure functions, exceptions and inlining and I 
> noticed that LDC seems to generate inferior code to GDC in the cases I 
> was considering.
> […]
> https://godbolt.org/g/WI12Bk

That's indeed a bit surprising. The generated IR looks like this:

---
define i64 @_D4test3fooFyAlZl({ i64, i64* } %a_arg) local_unnamed_addr 
#0 comdat {
   %.getAddressOf_dump.i2 = alloca { i64, i8* }, align 8 ; [#uses = 4, 
size/byte = 16]
   %.getAddressOf_dump.i = alloca { i64, i8* }, align 8 ; [#uses = 4, 
size/byte = 16]
   %a_arg.fca.1.extract = extractvalue { i64, i64* } %a_arg, 1 ; [#uses 
= 2]
   %1 = load i64, i64* %a_arg.fca.1.extract, align 8 ; [#uses = 2]
   %2 = bitcast { i64, i8* }* %.getAddressOf_dump.i to i8* ; [#uses = 2]
   call void @llvm.lifetime.start(i64 16, i8* %2)
   %3 = icmp eq i64 %1, 9223372036854775807        ; [#uses = 1]
   br i1 %3, label %if.i, label %_D4test4add1FNalZl.exit

[…]

_D4test4add1FNalZl.exit:                          ; preds = %0
   call void @llvm.lifetime.end(i64 16, i8* %2)
   %9 = load i64, i64* %a_arg.fca.1.extract, align 8 ; [#uses = 2]
   %10 = bitcast { i64, i8* }* %.getAddressOf_dump.i2 to i8* ; [#uses = 
2]
   call void @llvm.lifetime.start(i64 16, i8* %10)
   %11 = icmp eq i64 %9, 9223372036854775807       ; [#uses = 1]
   br i1 %11, label %if.i8, label %_D4test4add1FNalZl.exit9
---

There doesn't seem to be any stores which would potentially make %9 not 
foldable with %1. Tracking this down might be an interesting trip down 
the LLVM pass pipeline.

  — David