Use pragmas to work around MSVC x86_32 debug miscompile bug

Halide users reported this here: https://llvm.org/pr46176 I reported the issue to MSVC here: https://developercommunity.visualstudio.com/content/problem/1179643/msvc-copies-overaligned-non-trivially-copyable-par.html This codepath is apparently not covered by LLVM's unit tests, so I added coverage in a unit test. If we want to support this configuration going forward, it means that is in general not safe to pass a SmallVector<T, N> by value if alignof(T) is greater than 4. This doesn't appear to come up often because passing a SmallVector by value is inefficient and not idiomatic: it copies the inline storage. In this case, the SmallVector<LLT,4> is captured by value by a lambda, and the lambda is passed by value into std::function, and that's how we hit the bug. Differential Revision: https://reviews.llvm.org/D87475 NOTE from amaiorano@ : applied this LLVM patch (without the unittest) to SwiftShader. Bug: b/173512119 Change-Id: If036f61386b51f9b4a20427812b4ac92d901244d Reviewed-on: https://swiftshader-review.googlesource.com/c/SwiftShader/+/50469Tested-by: Antonio Maiorano <amaiorano@google.com> Reviewed-by: Nicolas Capens <nicolascapens@google.com>

Use pragmas to work around MSVC x86_32 debug miscompile bug
43fcd32b · Reid Kleckner · Antonio Maiorano · 50066a3e · 43fcd32b
Commit 43fcd32b authored Sep 10, 2020 by Reid Kleckner Committed by Antonio Maiorano Nov 18, 2020
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LegalityPredicates.cpp ...m-10.0/llvm/lib/CodeGen/GlobalISel/LegalityPredicates.cpp +11 -0

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--- a/third_party/llvm-10.0/llvm/lib/CodeGen/GlobalISel/LegalityPredicates.cpp
+++ b/third_party/llvm-10.0/llvm/lib/CodeGen/GlobalISel/LegalityPredicates.cpp
@@ -10,6 +10,17 @@
 //
 //===----------------------------------------------------------------------===//

+// Enable optimizations to work around MSVC debug mode bug in 32-bit:
+// https://developercommunity.visualstudio.com/content/problem/1179643/msvc-copies-overaligned-non-trivially-copyable-par.html
+// FIXME: Remove this when the issue is closed.
+#if defined(_MSC_VER) && !defined(__clang__) && defined(_M_IX86)
+// We have to disable runtime checks in order to enable optimizations. This is
+// done for the entire file because the problem is actually observed in STL
+// template functions.
+#pragma runtime_checks("", off)
+#pragma optimize("gs", on)
+#endif
+
 #include "llvm/CodeGen/GlobalISel/LegalizerInfo.h"

 using namespace llvm;