1 //===----------------------------------------------------------------------===//
2 // Representing sign/zero extension of function results
3 //===----------------------------------------------------------------------===//
5 Mar 25, 2009 - Initial Revision
7 Most ABIs specify that functions which return small integers do so in a
8 specific integer GPR. This is an efficient way to go, but raises the question:
9 if the returned value is smaller than the register, what do the high bits hold?
11 There are three (interesting) possible answers: undefined, zero extended, or
12 sign extended. The number of bits in question depends on the data-type that
13 the front-end is referencing (typically i1/i8/i16/i32).
15 Knowing the answer to this is important for two reasons: 1) we want to be able
16 to implement the ABI correctly. If we need to sign extend the result according
17 to the ABI, we really really do need to do this to preserve correctness. 2)
18 this information is often useful for optimization purposes, and we want the
19 mid-level optimizers to be able to process this (e.g. eliminate redundant
22 For example, lets pretend that X86 requires the caller to properly extend the
23 result of a return (I'm not sure this is the case, but the argument doesn't
24 depend on this). Given this, we should compile this:
27 short b() { return a(); }
38 An optimization example is that we should be able to eliminate the explicit
39 sign extension in this example:
43 return ((int)y() << 16) >> 16;
49 ;; movswl %ax, %eax -> not needed because eax is already sext'd
53 //===----------------------------------------------------------------------===//
54 // What we have right now.
55 //===----------------------------------------------------------------------===//
57 Currently, these sorts of things are modelled by compiling a function to return
58 the small type and a signext/zeroext marker is used. For example, we compile
61 define i32 @z() nounwind {
63 %0 = tail call signext i16 (...)* @y() nounwind
64 %1 = sext i16 %0 to i32
70 define signext i16 @b() nounwind {
72 %0 = tail call i32 (...)* @a() nounwind ; <i32> [#uses=1]
73 %retval12 = trunc i32 %0 to i16 ; <i16> [#uses=1]
77 This has some problems: 1) the actual precise semantics are really poorly
78 defined (see PR3779). 2) some targets might want the caller to extend, some
79 might want the callee to extend 3) the mid-level optimizer doesn't know the
80 size of the GPR, so it doesn't know that %0 is sign extended up to 32-bits
81 here, and even if it did, it could not eliminate the sext. 4) the code
82 generator has historically assumed that the result is extended to i32, which is
83 a problem on PIC16 (and is also probably wrong on alpha and other 64-bit
86 //===----------------------------------------------------------------------===//
88 //===----------------------------------------------------------------------===//
90 I suggest that we have the front-end fully lower out the ABI issues here to
91 LLVM IR. This makes it 100% explicit what is going on and means that there is
92 no cause for confusion. For example, the cases above should compile into:
94 define i32 @z() nounwind {
96 %0 = tail call i32 (...)* @y() nounwind
97 %1 = trunc i32 %0 to i16
98 %2 = sext i16 %1 to i32
101 define i32 @b() nounwind {
103 %0 = tail call i32 (...)* @a() nounwind
104 %retval12 = trunc i32 %0 to i16
105 %tmp = sext i16 %retval12 to i32
109 In this model, no functions will return an i1/i8/i16 (and on a x86-64 target
110 that extends results to i64, no i32). This solves the ambiguity issue, allows us
111 to fully describe all possible ABIs, and now allows the optimizers to reason
112 about and eliminate these extensions.
114 The one thing that is missing is the ability for the front-end and optimizer to
115 specify/infer the guarantees provided by the ABI to allow other optimizations.
116 For example, in the y/z case, since y is known to return a sign extended value,
117 the trunc/sext in z should be eliminable.
119 This can be done by introducing new sext/zext attributes which mean "I know
120 that the result of the function is sign extended at least N bits. Given this,
121 and given that it is stuck on the y function, the mid-level optimizer could
122 easily eliminate the extensions etc with existing functionality.
124 The major disadvantage of doing this sort of thing is that it makes the ABI
125 lowering stuff even more explicit in the front-end, and that we would like to
126 eventually move to having the code generator do more of this work. However,
127 the sad truth of the matter is that this is a) unlikely to happen anytime in
128 the near future, and b) this is no worse than we have now with the existing
131 C compilers fundamentally have to reason about the target in many ways.
132 This is ugly and horrible, but a fact of life.