[PATCH] D52550: [ARM] Check for sel intrinsic use in ARM CGP

[Sam Parker via Phabricator via llvm-commits]

samparker created this revision.
samparker added reviewers: efriedma, john.brawn, SjoerdMeijer.
Herald added a reviewer: javed.absar.
Herald added subscribers: chrib, kristof.beyls.

During initialisation, check whether the llvm.arm.sel intrinsic is used in the current module. If it has, we conservatively choose that it would be unsafe to generate the DSP add/sub instructions that will write to the GE flags - the same flags which sel will read.


https://reviews.llvm.org/D52550

Files:
  lib/Target/ARM/ARMCodeGenPrepare.cpp
  test/CodeGen/ARM/arm-cgp-sel.ll


Index: test/CodeGen/ARM/arm-cgp-sel.ll
===================================================================
--- /dev/null
+++ test/CodeGen/ARM/arm-cgp-sel.ll
@@ -0,0 +1,28 @@
+; RUN: llc -mtriple=thumbv7em %s -o - | FileCheck %s
+; RUN: llc -mtriple=thumbv8m.main -mcpu=cortex-m33 -arm-enable-scalar-dsp=true %s -o - | FileCheck %s
+; RUN: llc -mtriple=thumbv8m.main -mcpu=cortex-m33 -arm-enable-scalar-dsp=true -arm-enable-scalar-dsp-imms=true %s -o - | FileCheck %s
+
+define i32 @use_sel(i32 %a, i32 %b) {
+  %1 = call i32 @llvm.arm.sel(i32 %a, i32 %b)
+  ret i32 %1
+}
+
+; Test that because the sel intrinsic is used above, we don't generate a dsp
+; sub.
+; CHECK-LABEL: dsp_disable:
+; CHECK-NOT: usub
+; CHECK-NOT: ssub
+define void @dsp_disable(i8* %in, i8* %out, i8 %compare) {
+  %first = getelementptr inbounds i8, i8* %in, i32 0
+  %second = getelementptr inbounds i8, i8* %in, i32 1
+  %ld0 = load i8, i8* %first
+  %ld1 = load i8, i8* %second
+  %xor = xor i8 %ld0, -1
+  %cmp = icmp ult i8 %compare, %ld1
+  %select = select i1 %cmp, i8 %compare, i8 %xor
+  %sub = sub i8 %ld0, %select
+  store i8 %sub, i8* %out, align 1
+  ret void
+}
+
+declare i32 @llvm.arm.sel(i32, i32)
Index: lib/Target/ARM/ARMCodeGenPrepare.cpp
===================================================================
--- lib/Target/ARM/ARMCodeGenPrepare.cpp
+++ lib/Target/ARM/ARMCodeGenPrepare.cpp
@@ -136,6 +136,7 @@
   const ARMSubtarget *ST = nullptr;
   IRPromoter *Promoter = nullptr;
   std::set<Value*> AllVisited;
+  bool SafeToWriteGE = false;
 
   bool isSupportedValue(Value *V);
   bool isLegalToPromote(Value *V);
@@ -658,6 +659,12 @@
   if (!I)
     return false;
 
+  // Predicated on whether the sel intrinsic has been used within the module.
+  // If it has, conservatively choose to not generate any instructions that
+  // would overwrite the GE flags.
+  if (!SafeToWriteGE)
+    return false;
+
   // If promotion is not safe, can we use a DSP instruction to natively
   // handle the narrow type?
   if (!ST->hasDSP() || !EnableDSP || !isSupportedType(I))
@@ -789,6 +796,8 @@
 
 bool ARMCodeGenPrepare::doInitialization(Module &M) {
   Promoter = new IRPromoter(&M);
+  SafeToWriteGE =
+    Intrinsic::getDeclaration(&M, Intrinsic::arm_sel)->hasNUses(0);
   return false;
 }
 


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