[PATCH] D20167: [mips] Compact branch policy control for MIPSR6

[Simon Dardis via llvm-commits]

sdardis created this revision.
sdardis added subscribers: llvm-commits, vkalintiris.
sdardis set the repository for this revision to rL LLVM.
Herald added subscribers: sdardis, dsanders.

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.


Repository:
  rL LLVM

http://reviews.llvm.org/D20167

Files:
  lib/Target/Mips/MipsDelaySlotFiller.cpp
  test/CodeGen/Mips/compactbranches/compact-branch-policy.ll

Index: test/CodeGen/Mips/compactbranches/compact-branch-policy.ll
===================================================================
--- /dev/null
+++ test/CodeGen/Mips/compactbranches/compact-branch-policy.ll
@@ -0,0 +1,26 @@
+; 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
+; 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
Index: lib/Target/Mips/MipsDelaySlotFiller.cpp
===================================================================
--- lib/Target/Mips/MipsDelaySlotFiller.cpp
+++ lib/Target/Mips/MipsDelaySlotFiller.cpp
@@ -62,6 +62,24 @@
   cl::desc("Disallow MIPS delay filler to search backward."),
   cl::Hidden);
 
+enum CompactBranchPolicy {
+  CBNever,
+  CBOptimal,
+  CBAlways
+};
+
+static cl::opt<CompactBranchPolicy> MipsCompactBranchPolicy(
+  "mips-compact-branches",cl::Optional,
+  cl::init(CBOptimal),
+  cl::desc("MIPS Specific: Compact branch policy."),
+  cl::values(
+    clEnumValN(CBNever, "never", "Do not use compact branches if possible."),
+    clEnumValN(CBOptimal, "optimal", "Use compact branches where appropiate (default)."),
+    clEnumValN(CBAlways, "always", "Always use compact branches if possible."),
+    clEnumValEnd
+  )
+);
+
 namespace {
   typedef MachineBasicBlock::iterator Iter;
   typedef MachineBasicBlock::reverse_iterator ReverseIter;
@@ -563,14 +581,17 @@
     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() != CBAlways ||
+           !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 +617,9 @@
     // 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 != CBNever)) &&
+        TII->getEquivalentCompactForm(I)) {
       I = replaceWithCompactBranch(MBB, I, I->getDebugLoc());
       continue;
     }


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