[llvm] r269753 - [mips] Compact branch policy control for MIPSR6

[Simon Dardis via llvm-commits]

Author: sdardis
Date: Tue May 17 05:21:43 2016
New Revision: 269753

URL: http://llvm.org/viewvc/llvm-project?rev=269753&view=rev
Log:
[mips] Compact branch policy control for MIPSR6

This patch adds the commandline option -mips-compact-branches={never,optimal,always),
which controls how LLVM generates compact branches for MIPS targets. By
default, the compact branch policy is 'optimal' where LLVM will (hopefully)
pick the optimal branch for any situation. The 'never' policy will disable
the generation of compact branches and 'always' will generate compact branches
wherever possible.

Reviewers: dsanders

Differential Review: http://reviews.llvm.org/D20167

Added:
    llvm/trunk/test/CodeGen/Mips/compactbranches/compact-branch-policy.ll
Modified:
    llvm/trunk/lib/Target/Mips/MipsDelaySlotFiller.cpp

Modified: llvm/trunk/lib/Target/Mips/MipsDelaySlotFiller.cpp
URL: http://llvm.org/viewvc/llvm-project/llvm/trunk/lib/Target/Mips/MipsDelaySlotFiller.cpp?rev=269753&r1=269752&r2=269753&view=diff
==============================================================================
--- llvm/trunk/lib/Target/Mips/MipsDelaySlotFiller.cpp (original)
+++ llvm/trunk/lib/Target/Mips/MipsDelaySlotFiller.cpp Tue May 17 05:21:43 2016
@@ -62,6 +62,28 @@ static cl::opt<bool> DisableBackwardSear
   cl::desc("Disallow MIPS delay filler to search backward."),
   cl::Hidden);
 
+enum CompactBranchPolicy {
+  CB_Never,   ///< The policy 'never' may in some circumstances or for some
+              ///< ISAs not be absolutely adhered to.
+  CB_Optimal, ///< Optimal is the default and will produce compact branches
+              ///< when delay slots cannot be filled.
+  CB_Always   ///< 'always' may in some circumstances may not be
+              ///< absolutely adhered to there may not be a corresponding
+              ///< compact form of a branch.
+};
+
+static cl::opt<CompactBranchPolicy> MipsCompactBranchPolicy(
+  "mips-compact-branches",cl::Optional,
+  cl::init(CB_Optimal),
+  cl::desc("MIPS Specific: Compact branch policy."),
+  cl::values(
+    clEnumValN(CB_Never, "never", "Do not use compact branches if possible."),
+    clEnumValN(CB_Optimal, "optimal", "Use compact branches where appropiate (default)."),
+    clEnumValN(CB_Always, "always", "Always use compact branches if possible."),
+    clEnumValEnd
+  )
+);
+
 namespace {
   typedef MachineBasicBlock::iterator Iter;
   typedef MachineBasicBlock::reverse_iterator ReverseIter;
@@ -563,14 +585,17 @@ bool Filler::runOnMachineBasicBlock(Mach
     if (!DisableDelaySlotFiller && (TM.getOptLevel() != CodeGenOpt::None)) {
       bool Filled = false;
 
-      if (searchBackward(MBB, I)) {
-        Filled = true;
-      } else if (I->isTerminator()) {
-        if (searchSuccBBs(MBB, I)) {
+      if (MipsCompactBranchPolicy.getValue() != CB_Always ||
+           !TII->getEquivalentCompactForm(I)) {
+        if (searchBackward(MBB, I)) {
+          Filled = true;
+        } else if (I->isTerminator()) {
+          if (searchSuccBBs(MBB, I)) {
+            Filled = true;
+          }
+        } else if (searchForward(MBB, I)) {
           Filled = true;
         }
-      } else if (searchForward(MBB, I)) {
-        Filled = true;
       }
 
       if (Filled) {
@@ -596,8 +621,9 @@ bool Filler::runOnMachineBasicBlock(Mach
     // For MIPSR6 attempt to produce the corresponding compact (no delay slot)
     // form of the CTI. For indirect jumps this will not require inserting a
     // NOP and for branches will hopefully avoid requiring a NOP.
-    if ((InMicroMipsMode || STI.hasMips32r6()) &&
-         TII->getEquivalentCompactForm(I)) {
+    if ((InMicroMipsMode ||
+         (STI.hasMips32r6() && MipsCompactBranchPolicy != CB_Never)) &&
+        TII->getEquivalentCompactForm(I)) {
       I = replaceWithCompactBranch(MBB, I, I->getDebugLoc());
       continue;
     }

Added: llvm/trunk/test/CodeGen/Mips/compactbranches/compact-branch-policy.ll
URL: http://llvm.org/viewvc/llvm-project/llvm/trunk/test/CodeGen/Mips/compactbranches/compact-branch-policy.ll?rev=269753&view=auto
==============================================================================
--- llvm/trunk/test/CodeGen/Mips/compactbranches/compact-branch-policy.ll (added)
+++ llvm/trunk/test/CodeGen/Mips/compactbranches/compact-branch-policy.ll Tue May 17 05:21:43 2016
@@ -0,0 +1,28 @@
+; Check that -mips-compact-branches={never,optimal,always} is accepted and honoured.
+; RUN: llc -march=mips -mcpu=mips32r6 -mips-compact-branches=never < %s | FileCheck %s -check-prefix=NEVER
+; RUN: llc -march=mips -mcpu=mips32r6 -mips-compact-branches=optimal < %s | FileCheck %s -check-prefix=OPTIMAL
+; RUN: llc -march=mips -mcpu=mips32r6 -mips-compact-branches=always < %s | FileCheck %s -check-prefix=ALWAYS
+
+define i32 @l(i32 signext %a, i32 signext %b) {
+entry:
+  %add = add nsw i32 %b, %a
+  %cmp = icmp slt i32 %add, 100
+; NEVER: beq
+; OPTIMAL: beq
+; ALWAYS: beqzc
+; This nop is required for correct as having (j|b)al as the instruction
+; immediately following beqzc would cause a forbidden slot hazard.
+; ALWAYS: nop
+  br i1 %cmp, label %if.then, label %if.end
+
+if.then:                                          ; preds = %entry
+  %call = tail call i32 @k()
+  br label %if.end
+
+if.end:                                           ; preds = %entry, %if.then
+  %call.pn = phi i32 [ %call, %if.then ], [ -1, %entry ]
+  %c.0 = add nsw i32 %call.pn, %add
+  ret i32 %c.0
+}
+
+declare i32 @k() #1