[flang] Add -fcomplex-arithmetic= option and select complex division algorithm (#146641)

This patch adds an option to select the method for computing complex
number division. It uses `LoweringOptions` to determine whether to lower
complex division to a runtime function call or to MLIR's `complex.div`,
and `CodeGenOptions` to select the computation algorithm for
`complex.div`. The available option values and their corresponding
algorithms are as follows:
- `full`: Lower to a runtime function call. (Default behavior)
- `improved`: Lower to `complex.div` and expand to Smith's algorithm.
- `basic`: Lower to `complex.div` and expand to the algebraic algorithm.

See also the discussion in the following discourse post:
https://discourse.llvm.org/t/optimization-of-complex-number-division/83468

---------

Co-authored-by: Tarun Prabhu <tarunprabhu@gmail.com>

2 files changed, 23 insertions, 0 deletions

diff --git a/flang/lib/Frontend/CompilerInvocation.cpp b/flang/lib/Frontend/CompilerInvocation.cpp
index e5c6b1d..5455efd 100644
--- a/flang/lib/Frontend/CompilerInvocation.cpp
+++ b/flang/lib/Frontend/CompilerInvocation.cpp
@@ -485,6 +485,21 @@ static void parseCodeGenArgs(Fortran::frontend::CodeGenOptions &opts,
   }
 
   parseDoConcurrentMapping(opts, args, diags);
+
+  if (const llvm::opt::Arg *arg =
+          args.getLastArg(clang::driver::options::OPT_complex_range_EQ)) {
+    llvm::StringRef argValue = llvm::StringRef(arg->getValue());
+    if (argValue == "full") {
+      opts.setComplexRange(CodeGenOptions::ComplexRangeKind::CX_Full);
+    } else if (argValue == "improved") {
+      opts.setComplexRange(CodeGenOptions::ComplexRangeKind::CX_Improved);
+    } else if (argValue == "basic") {
+      opts.setComplexRange(CodeGenOptions::ComplexRangeKind::CX_Basic);
+    } else {
+      diags.Report(clang::diag::err_drv_invalid_value)
+          << arg->getAsString(args) << arg->getValue();
+    }
+  }
 }
 
 /// Parses all target input arguments and populates the target
@@ -1812,4 +1827,10 @@ void CompilerInvocation::setLoweringOptions() {
       .setNoSignedZeros(langOptions.NoSignedZeros)
       .setAssociativeMath(langOptions.AssociativeMath)
       .setReciprocalMath(langOptions.ReciprocalMath);
+
+  if (codegenOpts.getComplexRange() ==
+          CodeGenOptions::ComplexRangeKind::CX_Improved ||
+      codegenOpts.getComplexRange() ==
+          CodeGenOptions::ComplexRangeKind::CX_Basic)
+    loweringOpts.setComplexDivisionToRuntime(false);
 }
diff --git a/flang/lib/Frontend/FrontendActions.cpp b/flang/lib/Frontend/FrontendActions.cpp
index bf15def..b5f4f94 100644
--- a/flang/lib/Frontend/FrontendActions.cpp
+++ b/flang/lib/Frontend/FrontendActions.cpp
@@ -750,6 +750,8 @@ void CodeGenAction::generateLLVMIR() {
   if (ci.getInvocation().getLoweringOpts().getIntegerWrapAround())
     config.NSWOnLoopVarInc = false;
 
+  config.ComplexRange = opts.getComplexRange();
+
   // Create the pass pipeline
   fir::createMLIRToLLVMPassPipeline(pm, config, getCurrentFile());
   (void)mlir::applyPassManagerCLOptions(pm);