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annotate docs/ExtendedIntegerResults.txt @ 107:a03ddd01be7e

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author Kaito Tokumori <e105711@ie.u-ryukyu.ac.jp>
date Sun, 31 Jan 2016 17:34:49 +0900
parents 95c75e76d11b
children
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1 //===----------------------------------------------------------------------===//
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2 // Representing sign/zero extension of function results
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3 //===----------------------------------------------------------------------===//
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4
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5 Mar 25, 2009 - Initial Revision
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6
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7 Most ABIs specify that functions which return small integers do so in a
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8 specific integer GPR. This is an efficient way to go, but raises the question:
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9 if the returned value is smaller than the register, what do the high bits hold?
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10
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11 There are three (interesting) possible answers: undefined, zero extended, or
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12 sign extended. The number of bits in question depends on the data-type that
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13 the front-end is referencing (typically i1/i8/i16/i32).
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14
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15 Knowing the answer to this is important for two reasons: 1) we want to be able
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16 to implement the ABI correctly. If we need to sign extend the result according
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17 to the ABI, we really really do need to do this to preserve correctness. 2)
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18 this information is often useful for optimization purposes, and we want the
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19 mid-level optimizers to be able to process this (e.g. eliminate redundant
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20 extensions).
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21
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22 For example, lets pretend that X86 requires the caller to properly extend the
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23 result of a return (I'm not sure this is the case, but the argument doesn't
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24 depend on this). Given this, we should compile this:
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25
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26 int a();
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27 short b() { return a(); }
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28
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29 into:
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30
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31 _b:
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32 subl $12, %esp
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33 call L_a$stub
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34 addl $12, %esp
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35 cwtl
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36 ret
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37
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38 An optimization example is that we should be able to eliminate the explicit
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39 sign extension in this example:
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40
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41 short y();
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42 int z() {
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43 return ((int)y() << 16) >> 16;
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44 }
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45
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46 _z:
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47 subl $12, %esp
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48 call _y
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49 ;; movswl %ax, %eax -> not needed because eax is already sext'd
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50 addl $12, %esp
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51 ret
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52
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53 //===----------------------------------------------------------------------===//
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54 // What we have right now.
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55 //===----------------------------------------------------------------------===//
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56
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57 Currently, these sorts of things are modelled by compiling a function to return
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58 the small type and a signext/zeroext marker is used. For example, we compile
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59 Z into:
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60
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61 define i32 @z() nounwind {
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62 entry:
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63 %0 = tail call signext i16 (...)* @y() nounwind
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64 %1 = sext i16 %0 to i32
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65 ret i32 %1
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66 }
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67
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68 and b into:
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69
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70 define signext i16 @b() nounwind {
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71 entry:
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72 %0 = tail call i32 (...)* @a() nounwind ; <i32> [#uses=1]
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73 %retval12 = trunc i32 %0 to i16 ; <i16> [#uses=1]
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74 ret i16 %retval12
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75 }
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76
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77 This has some problems: 1) the actual precise semantics are really poorly
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78 defined (see PR3779). 2) some targets might want the caller to extend, some
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79 might want the callee to extend 3) the mid-level optimizer doesn't know the
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80 size of the GPR, so it doesn't know that %0 is sign extended up to 32-bits
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81 here, and even if it did, it could not eliminate the sext. 4) the code
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82 generator has historically assumed that the result is extended to i32, which is
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83 a problem on PIC16 (and is also probably wrong on alpha and other 64-bit
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84 targets).
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85
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86 //===----------------------------------------------------------------------===//
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87 // The proposal
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88 //===----------------------------------------------------------------------===//
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89
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90 I suggest that we have the front-end fully lower out the ABI issues here to
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91 LLVM IR. This makes it 100% explicit what is going on and means that there is
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92 no cause for confusion. For example, the cases above should compile into:
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93
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94 define i32 @z() nounwind {
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95 entry:
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96 %0 = tail call i32 (...)* @y() nounwind
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97 %1 = trunc i32 %0 to i16
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98 %2 = sext i16 %1 to i32
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99 ret i32 %2
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100 }
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101 define i32 @b() nounwind {
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102 entry:
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103 %0 = tail call i32 (...)* @a() nounwind
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104 %retval12 = trunc i32 %0 to i16
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105 %tmp = sext i16 %retval12 to i32
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106 ret i32 %tmp
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107 }
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108
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109 In this model, no functions will return an i1/i8/i16 (and on a x86-64 target
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110 that extends results to i64, no i32). This solves the ambiguity issue, allows us
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111 to fully describe all possible ABIs, and now allows the optimizers to reason
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112 about and eliminate these extensions.
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113
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114 The one thing that is missing is the ability for the front-end and optimizer to
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115 specify/infer the guarantees provided by the ABI to allow other optimizations.
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116 For example, in the y/z case, since y is known to return a sign extended value,
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117 the trunc/sext in z should be eliminable.
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118
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119 This can be done by introducing new sext/zext attributes which mean "I know
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120 that the result of the function is sign extended at least N bits. Given this,
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121 and given that it is stuck on the y function, the mid-level optimizer could
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122 easily eliminate the extensions etc with existing functionality.
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123
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124 The major disadvantage of doing this sort of thing is that it makes the ABI
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125 lowering stuff even more explicit in the front-end, and that we would like to
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126 eventually move to having the code generator do more of this work. However,
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127 the sad truth of the matter is that this is a) unlikely to happen anytime in
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128 the near future, and b) this is no worse than we have now with the existing
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129 attributes.
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130
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Kaito Tokumori <e105711@ie.u-ryukyu.ac.jp>
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131 C compilers fundamentally have to reason about the target in many ways.
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Kaito Tokumori <e105711@ie.u-ryukyu.ac.jp>
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132 This is ugly and horrible, but a fact of life.
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Kaito Tokumori <e105711@ie.u-ryukyu.ac.jp>
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133