[dxil] Proposal to add new debug printf dxil op

proposals/NNNN-debug-printf.md

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+# Dxil Debug Printf
+
+* Proposal: [NNNN](NNNN-debug-printf.md)
+* Author(s): [Jiao Lu](https://github.com/jiaolu)
+* Sponsor: TBD
+* Status: **Under Consideration**
+
+## Introduction
+
+Add new dxil op to allow c/c++ like *printf* intrinsic instructions can be
+generated in the dxil sourced from hlsl.
+
+## Motivation
+
+As the new shader models, i.e. RayTracing, RayQuery, Workgraph, and more complex
+algorithms and structure emerges in recent time, we meet many more requirements of 
+debugging capabilities of hlsl coding in game, application and driver development.
+
+The dxil counterpart spirv has a similar feature, 
+[NonSemantic.DebugPrintf extention](https://github.com/KhronosGroup/SPIRV-Registry/blob/main/nonsemantic/NonSemantic.DebugPrintf.asciidoc)
+to generate DebugPrintf spirvOp souced from hlsl/glsl. Based on the spirvOp
+extention, some vendor drivers and Valve lunarG has 
+[debug printf layer](https://github.com/KhronosGroup/Vulkan-ValidationLayers/blob/main/docs/debug_printf.md)
+to dump printf expression, "hlsl/glsl variables" into stdio or file.
+
+## Proposed solution
+
+The printf expression in hlsl mostly like this example.
+```c++ hlsl:
+
+const string str0= "str0";
+string str1 = "str1";
+
+void main() {
+    printf(str0);
+    printf(str1);
+    printf("Variables are: %d %d %.2f", 1u, 2u, 1.5f);
+}
+
+```
+
+The format specifier generally follow the c/c++ format specifiers as here, https://www.geeksforgeeks.org/format-specifiers-in-c/,
+Though how the final string representation after printf output is implementation dependent.
+
+
+DirectXCompiler at present can parse "printf" statement in hlsl as dx.hl.op 
+instructions,
+```
+dx.hl.op..void (i32, i8*, ...);
+```
+The printf format string, the second argument of the printf instruction of 
+dx.hl.op is a global *constant* variable or a GEP constant expression.
+
+The parsing is called from ast frontend into the 
+HandleTranslationUnit-->TranslatePrintf.The function *TranslatePrintf* itself
+ is empty implementation.
+
+The implementation of debug printf dxil op will be
+
+1) assign new dxil op code to the debug printf.
+2) Finished the TranslatePrintf implementation, create dxil op instruction with 
+a proper dxil op code, and replace dx.hl.op. the dxil op instruction function 
+will be a variable arguments function.
+
+
+## Detailed design
+
+1. The printf dxil op will be non-semantic, it does not affect final hlsl code/algorithm.
+Non-semantic dxil op code can be counted down from 0xffff, or 0xffffffff, it will give a hint to the client api
+to remove the non semantic dxil safely
+2. Add a option to enable printf dxil op generation to dxc, try to separate hlsl code for debugging
+and for production, if printf option is disabled, the printf in hlsl will be report a error
+3. We should not support dynamic string variable, a string variable content.
+retrieved from buffer. The string variable should be explicited defined and can 
+be retrieved directly/indirectly from global constant variable.
+4. The format string input to the dx.hl.op..void, could be llvm constant 
+expression, we need to retrieve global variable from the constant expression.
+5. The validation for the dxil overloading checking should be ignored. Because 
+of printf variable arguments, there is no definite function type can be validated.
+6. dxc does not valiate format specifier to the c/c++ format speicifer standard, or the matching relation between
+format specifier and argument. If the number and type don't match, they will produce undefined result from 
+client api, e.g. driver.