[torque]: Add constexpr keyword/types for compile-time evaluation
Torque expressions of type constexpr are evaluated at compile-time rather than runtime. They are backed by C++ types rather than TNode<X> types, so the macro functions that are called by generated C++ code expect values to be computed when the snapshot is generated rather than by TurboFan-generated code. Specifically, "if" statements can have a constexpr modifier. With this modifier, a type of "constexpr bool" is expected rather than "bool", and in that case instead of generating a CSA BranchIf, it generates a C++ "if (<bool expression>)" that generates code for only the true or false path based on the bool value at torque-execution (compile time) rather than generating both paths (including inserting phi nodes for variables modified on either branch at the re-merge at the end of the if) and dynamically dispatching to the true or false path during d8/Chrome/node.js execution (runtime) using a CSA BranchIf. Change-Id: I8238e25aaadbfc618847e04556e96a3949ea5a8d Reviewed-on: https://chromium-review.googlesource.com/1042085 Commit-Queue: Daniel Clifford <danno@chromium.org> Reviewed-by:Tobias Tebbi <tebbi@chromium.org> Cr-Commit-Position: refs/heads/master@{#53001}
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