[FastISel] Don't trivially kill extractvalues (PR49467)

All extractvalues of the same value at the same index will map to
the same register, so even if one specific extractvalue only has
one use, we should not mark it as a trivial kill, as there may be
more extractvalues later.

Fixes https://bugs.llvm.org/show_bug.cgi?id=49467.

Differential Revision: https://reviews.llvm.org/D98145

Author: nikic

llvm/include/llvm/CodeGen/FastISel.h

@@ -490,7 +490,10 @@ class FastISel {
   /// - \c Add has a constant operand.
   bool canFoldAddIntoGEP(const User *GEP, const Value *Add);
 
-  /// Test whether the given value has exactly one use.
+  /// Test whether the register associated with this value has exactly one use,
+  /// in which case that single use is killing. Note that multiple IR values
+  /// may map onto the same register, in which case this is not the same as
+  /// checking that an IR value has one use.
   bool hasTrivialKill(const Value *V);
 
   /// Create a machine mem operand from the given instruction.

llvm/lib/CodeGen/SelectionDAG/FastISel.cpp

@@ -261,12 +261,16 @@ bool FastISel::hasTrivialKill(const Value *V) {
     if (GEP->hasAllZeroIndices() && !hasTrivialKill(GEP->getOperand(0)))
       return false;
 
+  // Casts and extractvalues may be trivially coalesced by fast-isel.
+  if (I->getOpcode() == Instruction::BitCast ||
+      I->getOpcode() == Instruction::PtrToInt ||
+      I->getOpcode() == Instruction::IntToPtr ||
+      I->getOpcode() == Instruction::ExtractValue)
+    return false;
+
   // Only instructions with a single use in the same basic block are considered
   // to have trivial kills.
   return I->hasOneUse() &&
-         !(I->getOpcode() == Instruction::BitCast ||
-           I->getOpcode() == Instruction::PtrToInt ||
-           I->getOpcode() == Instruction::IntToPtr) &&
          cast<Instruction>(*I->user_begin())->getParent() == I->getParent();
 }
 

llvm/test/CodeGen/X86/pr49467.ll

@@ -0,0 +1,27 @@
+; NOTE: Assertions have been autogenerated by utils/update_llc_test_checks.py
+; RUN: llc -O0 -fast-isel -verify-machineinstrs -mtriple=x86_64 < %s | FileCheck %s
+
+declare { i8*, i64 } @get()
+
+declare void @use(i8*, i64)
+
+define void @test(i64* %p) nounwind {
+; CHECK-LABEL: test:
+; CHECK:       # %bb.0:
+; CHECK-NEXT:    pushq %rax
+; CHECK-NEXT:    movq %rdi, (%rsp) # 8-byte Spill
+; CHECK-NEXT:    callq get@PLT
+; CHECK-NEXT:    movq (%rsp), %rdi # 8-byte Reload
+; CHECK-NEXT:    movq %rdx, %rsi
+; CHECK-NEXT:    movq %rsi, (%rdi)
+; CHECK-NEXT:    # implicit-def: $rdi
+; CHECK-NEXT:    callq use@PLT
+; CHECK-NEXT:    popq %rax
+; CHECK-NEXT:    retq
+  %struct = call { i8*, i64 } @get()
+  %struct.1 = extractvalue { i8*, i64 } %struct, 1
+  store i64 %struct.1, i64* %p, align 8
+  %struct.2 = extractvalue { i8*, i64 } %struct, 1
+  call void @use(i8* undef, i64 %struct.2)
+  ret void
+}