[CGP] Permit tail call optimization on undefined return value

We may freely allow tail call optzs on undef values as well.

Fixes: #82387.

Author: antoniofrighetto

llvm/lib/CodeGen/CodeGenPrepare.cpp

@@ -2686,8 +2686,9 @@ bool CodeGenPrepare::dupRetToEnableTailCallOpts(BasicBlock *BB,
             attributesPermitTailCall(F, CI, RetI, *TLI)) {
           // Either we return void or the return value must be the first
           // argument of a known intrinsic or library function.
-          if (!V || (isIntrinsicOrLFToBeTailCalled(TLInfo, CI) &&
-                     V == CI->getArgOperand(0))) {
+          if (!V || isa<UndefValue>(V) ||
+              (isIntrinsicOrLFToBeTailCalled(TLInfo, CI) &&
+               V == CI->getArgOperand(0))) {
             TailCallBBs.push_back(Pred);
           }
         }

llvm/test/CodeGen/AArch64/addsub.ll

@@ -662,17 +662,13 @@ define dso_local i32 @_extract_crng_crng() {
 ; CHECK-NEXT:    cmn x8, #1272
 ; CHECK-NEXT:    b.pl .LBB36_3
 ; CHECK-NEXT:  .LBB36_2: // %if.then
-; CHECK-NEXT:    str x30, [sp, #-16]! // 8-byte Folded Spill
-; CHECK-NEXT:    .cfi_def_cfa_offset 16
-; CHECK-NEXT:    .cfi_offset w30, -16
 ; CHECK-NEXT:    adrp x8, primary_crng
 ; CHECK-NEXT:    ldr w8, [x8, :lo12:primary_crng]
 ; CHECK-NEXT:    cmp w8, #0
 ; CHECK-NEXT:    adrp x8, input_pool
 ; CHECK-NEXT:    add x8, x8, :lo12:input_pool
 ; CHECK-NEXT:    csel x0, xzr, x8, eq
-; CHECK-NEXT:    bl crng_reseed
-; CHECK-NEXT:    ldr x30, [sp], #16 // 8-byte Folded Reload
+; CHECK-NEXT:    b crng_reseed
 ; CHECK-NEXT:  .LBB36_3: // %if.end
 ; CHECK-NEXT:    ret
 entry:

llvm/test/CodeGen/AArch64/callbr-asm-obj-file.ll

@@ -40,7 +40,7 @@ declare dso_local i32 @g(...) local_unnamed_addr
 declare dso_local i32 @i(...) local_unnamed_addr
 
 ; CHECK-LABEL: <test2>:
-; CHECK:         bl {{.*}} <test2+0x18>
+; CHECK:         b {{.*}} <test2+0x1c>
 ; CHECK-LABEL: <$d.5>:
 ; CHECK-LABEL: <$x.6>:
 ; CHECK-NEXT:    b {{.*}} <test2+0x18>

llvm/test/CodeGen/RISCV/pr51206.ll

@@ -27,16 +27,12 @@ define signext i32 @wobble() nounwind {
 ; CHECK-NEXT:    lui a2, %hi(global.3)
 ; CHECK-NEXT:    li a3, 5
 ; CHECK-NEXT:    sw a1, %lo(global.3)(a2)
-; CHECK-NEXT:    bltu a0, a3, .LBB0_2
-; CHECK-NEXT:  # %bb.1: # %bb10
-; CHECK-NEXT:    addi sp, sp, -16
-; CHECK-NEXT:    sd ra, 8(sp) # 8-byte Folded Spill
-; CHECK-NEXT:    call quux
-; CHECK-NEXT:    ld ra, 8(sp) # 8-byte Folded Reload
-; CHECK-NEXT:    addi sp, sp, 16
-; CHECK-NEXT:  .LBB0_2: # %bb12
+; CHECK-NEXT:    bgeu a0, a3, .LBB0_2
+; CHECK-NEXT:  # %bb.1: # %bb12
 ; CHECK-NEXT:    li a0, 0
 ; CHECK-NEXT:    ret
+; CHECK-NEXT:  .LBB0_2: # %bb10
+; CHECK-NEXT:    tail quux
 bb:
   %tmp = load i8, ptr @global, align 1
   %tmp1 = zext i8 %tmp to i32

llvm/test/CodeGen/X86/tailcall-cgp-dup.ll

@@ -339,7 +339,7 @@ return:
 
 define ptr @strcpy_illegal_tailc(ptr %dest, i64 %sz, ptr readonly returned %src) nounwind {
 ; CHECK-LABEL: strcpy_illegal_tailc:
-; CHECK:       ## %bb.0:
+; CHECK:       ## %bb.0: ## %entry
 ; CHECK-NEXT:    pushq %rbx
 ; CHECK-NEXT:    movq %rdx, %rbx
 ; CHECK-NEXT:    testq %rsi, %rsi
@@ -351,6 +351,7 @@ define ptr @strcpy_illegal_tailc(ptr %dest, i64 %sz, ptr readonly returned %src)
 ; CHECK-NEXT:    movq %rbx, %rax
 ; CHECK-NEXT:    popq %rbx
 ; CHECK-NEXT:    retq
+entry:
   %cmp = icmp eq i64 %sz, 0
   br i1 %cmp, label %return, label %if.then
 
@@ -362,8 +363,63 @@ return:
   ret ptr %src
 }
 
+@i = global i32 0, align 4
+
+define i32 @undef_tailc() nounwind {
+; CHECK-LABEL: undef_tailc:
+; CHECK:       ## %bb.0: ## %entry
+; CHECK-NEXT:    cmpl $0, _i(%rip)
+; CHECK-NEXT:    jne _qux ## TAILCALL
+; CHECK-NEXT:  ## %bb.1: ## %return
+; CHECK-NEXT:    retq
+entry:
+  %val = load i32, ptr @i, align 4
+  %cmp = icmp eq i32 %val, 0
+  br i1 %cmp, label %return, label %if.then
+
+if.then:
+  %rv_unused = tail call i32 @qux()
+  br label %return
+
+return:
+  ret i32 undef
+}
+
+define i32 @undef_and_known_tailc() nounwind {
+; CHECK-LABEL: undef_and_known_tailc:
+; CHECK:       ## %bb.0: ## %entry
+; CHECK-NEXT:    movl _i(%rip), %eax
+; CHECK-NEXT:    cmpl $5, %eax
+; CHECK-NEXT:    je _qux ## TAILCALL
+; CHECK-NEXT:  ## %bb.1: ## %entry
+; CHECK-NEXT:    cmpl $2, %eax
+; CHECK-NEXT:    je _quux ## TAILCALL
+; CHECK-NEXT:  ## %bb.2: ## %return
+; CHECK-NEXT:    retq
+entry:
+  %val = load i32, ptr @i, align 4
+  switch i32 %val, label %return [
+    i32 2, label %case_2
+    i32 5, label %case_5
+  ]
+
+case_2:
+  %rv_unused = tail call i32 @quux()
+  br label %return
+
+case_5:
+  %rv = tail call i32 @qux()
+  br label %return
+
+return:
+  %phi = phi i32 [ undef, %case_2 ], [ %rv, %case_5 ], [ undef, %entry ]
+  ret i32 %phi
+}
+
 declare void @llvm.memcpy.p0.p0.i64(ptr noalias nocapture writeonly, ptr noalias nocapture readonly, i64, i1)
 declare void @llvm.memset.p0.i64(ptr nocapture writeonly, i8, i64, i1)
 declare noalias ptr @malloc(i64)
 declare ptr @strcpy(ptr noalias returned writeonly, ptr noalias nocapture readonly)
 declare ptr @baz(ptr, ptr)
+declare i32 @qux()
+declare i32 @quux()