[RISCV] Add backend command line options to control function and loop alignment for testing.

[craig.topper]

Summary

Clang doesn't support -falign-loops or -falign-functions. This
patch adds backend options to emulate them for testing. The
names were copied from x86. In the future we may want subtarget
based defaults, but I'm leaving that until we need it.

[MaskRay]

(clang supports one form: -falign-functions=N . I am working on a patch to add -falign-loops=N )

[craig.topper]

In D106570#2900392, @MaskRay wrote:

(clang supports one form: -falign-functions=N . I am working on a patch to add -falign-loops=N )

Oops. I guess I didn't read the list of ignored align options very closely to see that it was jumps, labels, and loops that weren't supported, and not functions.

[craig.topper]

Abandon since clang now supports -falign-loops

llvm/lib/Target/RISCV/RISCVISelLowering.cpp

 //===-- RISCVISelLowering.cpp - RISCV DAG Lowering Implementation  --------===//
 //
 // Part of the LLVM Project, under the Apache License v2.0 with LLVM Exceptions.
 // See https://llvm.org/LICENSE.txt for license information.
 // SPDX-License-Identifier: Apache-2.0 WITH LLVM-exception
 //
 //===----------------------------------------------------------------------===//
 //

 #include "llvm/Support/raw_ostream.h"
 
 using namespace llvm;
 
 #define DEBUG_TYPE "riscv-lower"
 
 STATISTIC(NumTailCalls, "Number of tail calls");
 
+static cl::opt<int> ExperimentalPrefLoopAlignment(
+    "riscv-experimental-pref-loop-alignment", cl::init(0),
+    cl::desc(
+        "Sets the preferable loop alignment for experiments (as log2 bytes)"
+        "(the last riscv-experimental-pref-loop-alignment bits"
+        " of the loop header PC will be 0)."),
+    cl::Hidden);
+
+static cl::opt<int> ExperimentalPrefFunctionAlignment(
+    "riscv-experimental-pref-function-alignment", cl::init(0),
+    cl::desc(
+        "Sets the preferable function alignment for experiments (as log2 bytes)"
+        "(the last riscv-experimental-pref-loop-alignment bits"
+        " of the loop header PC will be 0)."),
+    cl::Hidden);
+
 RISCVTargetLowering::RISCVTargetLowering(const TargetMachine &TM,
                                          const RISCVSubtarget &STI)
     : TargetLowering(TM), Subtarget(STI) {
 
   if (Subtarget.isRV32E())
     report_fatal_error("Codegen not yet implemented for RV32E");
 
   RISCVABI::ABI ABI = Subtarget.getTargetABI();
if (Subtarget.useRVVForFixedLengthVectors()) {
       setOperationAction(ISD::BITCAST, MVT::f32, Custom);
       setOperationAction(ISD::BITCAST, MVT::f64, Custom);
     }
   }
 
   // Function alignments.
   const Align FunctionAlignment(Subtarget.hasStdExtC() ? 2 : 4);
   setMinFunctionAlignment(FunctionAlignment);
-  setPrefFunctionAlignment(FunctionAlignment);
+  setPrefFunctionAlignment(std::max(
+      FunctionAlignment, Align(1 << ExperimentalPrefFunctionAlignment)));
+  setPrefLoopAlignment(Align(1 << ExperimentalPrefLoopAlignment));
 
   setMinimumJumpTableEntries(5);
 
   // Jumps are expensive, compared to logic
   setJumpIsExpensive();
 
   // We can use any register for comparisons
   setHasMultipleConditionRegisters();

llvm/test/CodeGen/RISCV/align.ll

 ; RUN: llc -mtriple=riscv32 -verify-machineinstrs < %s \
 ; RUN:   | FileCheck %s -check-prefix=RV32I
 ; RUN: llc -mtriple=riscv32 -mattr=+c -verify-machineinstrs < %s \
 ; RUN:   | FileCheck %s -check-prefix=RV32C
+; RUN: llc -mtriple=riscv32 -verify-machineinstrs < %s \
+; RUN:   -riscv-experimental-pref-function-alignment=3 | FileCheck %s \
+; RUN:   -check-prefix=ALIGN3
+; RUN: llc -mtriple=riscv32 -mattr=+c -verify-machineinstrs < %s \
+; RUN:   -riscv-experimental-pref-function-alignment=3 | FileCheck %s \
+; RUN:   -check-prefix=ALIGN3
+; RUN: llc -mtriple=riscv32 -verify-machineinstrs < %s \
+; RUN:   -riscv-experimental-pref-loop-alignment=3 | FileCheck %s \
+; RUN:   -check-prefix=LOOPALIGN
 
 define void @foo() {
 ;RV32I: .p2align 2
 ;RV32I: foo:
 ;RV32C: .p2align 1
 ;RV32C: foo:
+;ALIGN3: .p2align 3
+;ALIGN3: foo:
 entry:
   ret void
 }
+
+define void @bar(i32 %n) {
+;LOOPALIGN-LABEL: bar:
+;LOOPALIGN: .p2align        3
+;LOOPALIGN-NEXT: .LBB1_1:
+entry:
+  br label %loop
+
+loop:
+  %loop.iv = phi i32 [0, %entry], [%loop.iv.next, %loop]
+  call void @foo()
+  %loop.iv.next = add i32 %loop.iv, 1
+  %loop.cond = icmp ne i32 %loop.iv.next, %n
+  br i1 %loop.cond, label %loop, label %exit
+
+exit:
+  ret void
+}