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<h3>
Ben Gamari pushed to branch wip/T16742
at <a href="https://gitlab.haskell.org/ghc/ghc">Glasgow Haskell Compiler / GHC</a>
</h3>
<h4>
Commits:
</h4>
<ul>
<li>
<strong><a href="https://gitlab.haskell.org/ghc/ghc/commit/87b213520d22de4d0bca831c0ac75f8a8468ca59">87b21352</a></strong>
<div>
<span>by Ben Gamari</span>
<i>at 2019-06-10T13:32:36Z</i>
</div>
<pre class="commit-message" style="white-space: pre-wrap; margin: 0;">PrelRules: Don't break let/app invariant in shiftRule
Previously shiftRule would rewrite as invalid shift like
```
let x = I# (uncheckedIShiftL# n 80)
in ...
```
to
```
let x = I# (error "invalid shift")
in ...
```
However, this breaks the let/app invariant as `error` is not
okay-for-speculation. There isn't an easy way to avoid this so let's not
try. Instead we just take advantage of the undefined nature of invalid
shifts and return zero.
</pre>
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<h4>1 changed file:</h4>
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<a href="#844514241b3e1a009f66bf594cbb57fef61482fb">
compiler/prelude/PrelRules.hs
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<h4>Changes:</h4>
<li id="844514241b3e1a009f66bf594cbb57fef61482fb">
<a href="https://gitlab.haskell.org/ghc/ghc/commit/87b213520d22de4d0bca831c0ac75f8a8468ca59#844514241b3e1a009f66bf594cbb57fef61482fb"><strong>compiler/prelude/PrelRules.hs</strong></a>
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<td class="line_content match " style="padding-left: 0.5em; padding-right: 0.5em; color: rgba(0,0,0,0.3);" bgcolor="#fafafa">@@ -475,8 +475,7 @@ shiftRule shift_op</td>
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<pre style="margin: 0;"> <span id="LC475" class="line" lang="haskell"> <span class="o" style="font-weight: 600;">-></span> <span class="n" style="color: #333;">return</span> <span class="n" style="color: #333;">e1</span></span>
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<pre style="margin: 0;"> <span id="LC476" class="line" lang="haskell"> <span class="c1" style="color: #998; font-style: italic;">-- See Note [Guarding against silly shifts]</span></span>
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<pre style="margin: 0;"> <span id="LC477" class="line" lang="haskell"> <span class="o" style="font-weight: 600;">|</span> <span class="n" style="color: #333;">shift_len</span> <span class="o" style="font-weight: 600;"><</span> <span class="mi" style="color: #099;">0</span> <span class="o" style="font-weight: 600;">||</span> <span class="n" style="color: #333;">shift_len</span> <span class="o" style="font-weight: 600;">></span> <span class="n" style="color: #333;">wordSizeInBits</span> <span class="n" style="color: #333;">dflags</span></span>
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<pre style="margin: 0;">-<span id="LC478" class="line" lang="haskell"> <span class="o" style="font-weight: 600;">-></span> <span class="n" style="color: #333;">return</span> <span class="o" style="font-weight: 600;">$</span> <span class="n" style="color: #333;">mkRuntimeErrorApp</span> <span class="n" style="color: #333;">rUNTIME_ERROR_ID</span> <span class="n" style="color: #333;">wordPrimTy</span></span>
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<pre style="margin: 0;">-<span id="LC479" class="line" lang="haskell"> <span class="p">(</span><span class="s" style="color: #d14;">"Bad shift length "</span> <span class="o" style="font-weight: 600;">++</span> <span class="n" style="color: #333;">show</span> <span class="n" style="color: #333;">shift_len</span><span class="p">)</span></span>
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<pre style="margin: 0;">+<span id="LC478" class="line" lang="haskell"> <span class="o" style="font-weight: 600;">-></span> <span class="n" style="color: #333;">return</span> <span class="o" style="font-weight: 600;">$</span> <span class="n" style="color: #333;">mkLitNumberWrap</span> <span class="n" style="color: #333;">dflags</span> <span class="n" style="color: #333;">nt</span> <span class="mi" style="color: #099;">0</span> <span class="p">(</span><span class="n" style="color: #333;">exprType</span> <span class="n" style="color: #333;">e1</span><span class="p">)</span></span>
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<pre style="margin: 0;"> <span id="LC479" class="line" lang="haskell"></span>
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<pre style="margin: 0;"> <span id="LC480" class="line" lang="haskell"> <span class="c1" style="color: #998; font-style: italic;">-- Do the shift at type Integer, but shift length is Int</span></span>
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<pre style="margin: 0;"> <span id="LC481" class="line" lang="haskell"> <span class="kt" style="color: #458; font-weight: 600;">Lit</span> <span class="p">(</span><span class="kt" style="color: #458; font-weight: 600;">LitNumber</span> <span class="n" style="color: #333;">nt</span> <span class="n" style="color: #333;">x</span> <span class="n" style="color: #333;">t</span><span class="p">)</span></span>
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<td class="line_content match " style="padding-left: 0.5em; padding-right: 0.5em; color: rgba(0,0,0,0.3);" bgcolor="#fafafa">@@ -701,7 +700,23 @@ can't constant fold it, but if it gets to the assember we get</td>
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<pre style="margin: 0;"> <span id="LC700" class="line" lang="haskell"><span class="cm" style="color: #998; font-style: italic;"> Error: operand type mismatch for `shl'</span></span>
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<pre style="margin: 0;"> <span id="LC701" class="line" lang="haskell"></span>
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<pre style="margin: 0;"> <span id="LC702" class="line" lang="haskell"><span class="cm" style="color: #998; font-style: italic;">So the best thing to do is to rewrite the shift with a call to error,</span></span>
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<pre style="margin: 0;">-<span id="LC704" class="line" lang="haskell"><span class="cm" style="color: #998; font-style: italic;">when the second arg is stupid.</span></span>
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<pre style="margin: 0;">+<span id="LC703" class="line" lang="haskell"><span class="cm" style="color: #998; font-style: italic;">when the second arg is large. However, in general we cannot do this; consider</span></span>
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<pre style="margin: 0;">+<span id="LC704" class="line" lang="haskell"><span class="cm" style="color: #998; font-style: italic;">this case</span></span>
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<pre style="margin: 0;">+<span id="LC706" class="line" lang="haskell"><span class="cm" style="color: #998; font-style: italic;"> let x = I# (uncheckedIShiftL# n 80)</span></span>
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<pre style="margin: 0;">+<span id="LC707" class="line" lang="haskell"><span class="cm" style="color: #998; font-style: italic;"> in ...</span></span>
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<pre style="margin: 0;">+<span id="LC708" class="line" lang="haskell"></span>
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709
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<pre style="margin: 0;">+<span id="LC709" class="line" lang="haskell"><span class="cm" style="color: #998; font-style: italic;">Here x contains an invalid shift and consequently we would like to rewrite it</span></span>
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710
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<pre style="margin: 0;">+<span id="LC710" class="line" lang="haskell"><span class="cm" style="color: #998; font-style: italic;">as follows:</span></span>
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711
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<pre style="margin: 0;">+<span id="LC711" class="line" lang="haskell"></span>
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712
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<pre style="margin: 0;">+<span id="LC712" class="line" lang="haskell"><span class="cm" style="color: #998; font-style: italic;"> let x = I# (error "invalid shift)</span></span>
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713
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<pre style="margin: 0;">+<span id="LC713" class="line" lang="haskell"><span class="cm" style="color: #998; font-style: italic;"> in ...</span></span>
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714
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<pre style="margin: 0;">+<span id="LC714" class="line" lang="haskell"></span>
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715
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<pre style="margin: 0;">+<span id="LC715" class="line" lang="haskell"><span class="cm" style="color: #998; font-style: italic;">This was originally done in the fix to #16449 but this breaks the let/app</span></span>
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716
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<pre style="margin: 0;">+<span id="LC716" class="line" lang="haskell"><span class="cm" style="color: #998; font-style: italic;">invariant (see Note [CoreSyn let/app invariant] in CoreSyn) as noted in #16742.</span></span>
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<pre style="margin: 0;">+<span id="LC717" class="line" lang="haskell"><span class="cm" style="color: #998; font-style: italic;">Consequently we instead take advantage of the fact that large shifts are</span></span>
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718
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<pre style="margin: 0;">+<span id="LC718" class="line" lang="haskell"><span class="cm" style="color: #998; font-style: italic;">undefined behavior and transform the invalid shift into an "obviously</span></span>
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719
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<pre style="margin: 0;">+<span id="LC719" class="line" lang="haskell"><span class="cm" style="color: #998; font-style: italic;">incorrect" value.</span></span>
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705
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720
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<pre style="margin: 0;"> <span id="LC720" class="line" lang="haskell"></span>
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706
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721
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<pre style="margin: 0;"> <span id="LC721" class="line" lang="haskell"><span class="cm" style="color: #998; font-style: italic;">There are two cases:</span></span>
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707
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722
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<pre style="margin: 0;"> <span id="LC722" class="line" lang="haskell"></span>
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