Auto merge of #39456 - nagisa:mir-switchint-everywhere, r=nikomatsakis

[MIR] SwitchInt Everywhere

Something I've been meaning to do for a very long while. This PR essentially gets rid of 3 kinds of conditional branching and only keeps the most general one - `SwitchInt`. Primary benefits are such that dealing with MIR now does not involve dealing with 3 different ways to do conditional control flow. On the other hand, constructing a `SwitchInt` currently requires more code than what previously was necessary to build an equivalent `If` terminator. Something trivially "fixable" with some constructor methods somewhere (MIR needs stuff like that badly in general).

Some timings (tl;dr: slightly faster^1 (unexpected), but also uses slightly more memory at peak (expected)):

^1: Not sure if the speed benefits are because of LLVM liking the generated code better or the compiler itself getting compiled better. Either way, its a net benefit. The CORE and SYNTAX timings done for compilation without optimisation.

```
AFTER:
Building stage1 std artifacts (x86_64-unknown-linux-gnu -> x86_64-unknown-linux-gnu)
    Finished release [optimized] target(s) in 31.50 secs
    Finished release [optimized] target(s) in 31.42 secs
Building stage1 compiler artifacts (x86_64-unknown-linux-gnu -> x86_64-unknown-linux-gnu)
    Finished release [optimized] target(s) in 439.56 secs
    Finished release [optimized] target(s) in 435.15 secs

CORE: 99% (24.81 real, 0.13 kernel, 24.57 user); 358536k resident
CORE: 99% (24.56 real, 0.15 kernel, 24.36 user); 359168k resident
SYNTAX: 99% (49.98 real, 0.48 kernel, 49.42 user); 653416k resident
SYNTAX: 99% (50.07 real, 0.58 kernel, 49.43 user); 653604k resident

BEFORE:
Building stage1 std artifacts (x86_64-unknown-linux-gnu -> x86_64-unknown-linux-gnu)
    Finished release [optimized] target(s) in 31.84 secs
Building stage1 compiler artifacts (x86_64-unknown-linux-gnu -> x86_64-unknown-linux-gnu)
    Finished release [optimized] target(s) in 451.17 secs

CORE: 99% (24.66 real, 0.20 kernel, 24.38 user); 351096k resident
CORE: 99% (24.36 real, 0.17 kernel, 24.18 user); 352284k resident
SYNTAX: 99% (52.24 real, 0.56 kernel, 51.66 user); 645544k resident
SYNTAX: 99% (51.55 real, 0.48 kernel, 50.99 user); 646428k resident
```

cc @nikomatsakis @eddyb

1 files changed, 6 insertions, 0 deletions

diff --git a/src/rustllvm/RustWrapper.cpp b/src/rustllvm/RustWrapper.cpp
index 40432735911..e89f48b4105 100644
--- a/src/rustllvm/RustWrapper.cpp
+++ b/src/rustllvm/RustWrapper.cpp
@@ -1316,6 +1316,12 @@ extern "C" LLVMRustVisibility LLVMRustGetVisibility(LLVMValueRef V) {
   return toRust(LLVMGetVisibility(V));
 }
 
+// Oh hey, a binding that makes sense for once? (because LLVM’s own do not)
+extern "C" LLVMValueRef LLVMRustBuildIntCast(LLVMBuilderRef B, LLVMValueRef Val,
+                                             LLVMTypeRef DestTy, bool isSigned) {
+  return wrap(unwrap(B)->CreateIntCast(unwrap(Val), unwrap(DestTy), isSigned, ""));
+}
+
 extern "C" void LLVMRustSetVisibility(LLVMValueRef V,
                                       LLVMRustVisibility RustVisibility) {
   LLVMSetVisibility(V, fromRust(RustVisibility));