[PATCH] Refactor sameNoopInput (determining tail-call legitimacy)

[Duncan Sands]

Hi Tim,

On 25/07/13 14:19, Tim Northover wrote:
> This change came about primarily because of two issues in the existing code. Niether of:
>
> define i64 @test1(i64 %val) {
>    %in = trunc i64 %val to i32
>    tail call i32 @ret32(i32 returned %in)
>    ret i64 %val
> }
>
> define i64 @test2(i64 %val) {
>    tail call i32 @ret32(i32 returned undef)
>    ret i32 42
> }
>
> should be tail calls,

why not?

Ciao, Duncan.

  and the function sameNoopInput is responsible. The main problem is that it is 
completely symmetric in the "tail call" and "ret" value, but in reality 
different things are allowed on each side.
>
> For these cases:
> 1. Any truncation should lead to a larger value being generated by "tail call" than needed by "ret".
> 2. Undef should only be allowed as a source for ret, not as a result of the call.
>
> Along the way I noticed that a mismatch between what this function treats as a valid truncation and what the backends see can lead to invalid calls as well (see x86-32 test case).
>
> This patch refactors the code so that instead of being based primarily on values which it recurses into when necessary, it starts by inspecting the type and considers each fundamental slot that the backend will see in turn. For example, given a pathological function that returned {{}, {{}, i32, {}}, i32} we would consider each "real" i32 in turn, and ask if it passes through unchanged. This is much closer to what the backend sees as a result of ComputeValueVTs.
>
> Aside from the bug fixes, this eliminates the recursion that's going on and, I believe, makes the bulk of the code significantly easier to understand. The trade-off is the nasty iterators needed to find the real types inside a returned value.
>
>
> http://llvm-reviews.chandlerc.com/D1216
>
> Files:
>    lib/CodeGen/Analysis.cpp
>    test/CodeGen/X86/returned-trunc-tail-calls.ll
>    test/CodeGen/X86/tail-call-legality.ll
>
>
>
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