[AMDGPU] si-peephole-sdwa: Disable V_CNDMASK_B32 conversion with sext (#140760)

The sext modifier on an operand of V_CNDMASK_B32_sdwa gets erroneously
turned into a neg modifier in the assembly output.

As a workaround, to avoid miscompilation, this patch disables the
conversion of V_CNDMASK_B32 to the SDWA form if any operand uses an sext
modifier.

Fixes #138766.

---------

Co-authored-by: Matt Arsenault <[email protected]>

Author: frederik-h

llvm/lib/Target/AMDGPU/SIPeepholeSDWA.cpp

@@ -430,6 +430,21 @@ bool SDWASrcOperand::convertToSDWA(MachineInstr &MI, const SIInstrInfo *TII) {
   case AMDGPU::V_CVT_PK_F32_BF8_sdwa:
     // Does not support input modifiers: noabs, noneg, nosext.
     return false;
+  case AMDGPU::V_CNDMASK_B32_sdwa:
+    // SISrcMods uses the same bitmask for SEXT and NEG modifiers and
+    // hence the compiler can only support one type of modifier for
+    // each SDWA instruction.  For V_CNDMASK_B32_sdwa, this is NEG
+    // since its operands get printed using
+    // AMDGPUInstPrinter::printOperandAndFPInputMods which produces
+    // the output intended for NEG if SEXT is set.
+    //
+    // The ISA does actually support both modifiers on most SDWA
+    // instructions.
+    //
+    // FIXME Accept SEXT here after fixing this issue.
+    if (Sext)
+      return false;
+    break;
   }
 
   // Find operand in instruction that matches source operand and replace it with

llvm/test/CodeGen/AMDGPU/sdwa-peephole-cndmask-sext.ll

@@ -0,0 +1,21 @@
+; NOTE: Assertions have been autogenerated by utils/update_llc_test_checks.py UTC_ARGS: --version 5
+; RUN: llc -mtriple=amdgcn-amd-amdhsa -mcpu=gfx942 < %s | FileCheck %s
+
+; FIXME The sext modifier is turned into a neg modifier in the asm output
+
+define i32 @test_select_on_sext_sdwa(i8 %x, i32 %y, i1 %cond)  {
+; CHECK-LABEL: test_select_on_sext_sdwa:
+; CHECK:       ; %bb.0:
+; CHECK-NEXT:    s_waitcnt vmcnt(0) expcnt(0) lgkmcnt(0)
+; CHECK-NEXT:    v_and_b32_e32 v2, 1, v2
+; CHECK-NEXT:    v_cmp_eq_u32_e32 vcc, 1, v2
+; CHECK-NEXT:    v_bfe_i32 v0, v0, 0, 8
+; CHECK-NEXT:    s_nop 0
+; CHECK-NEXT:    v_cndmask_b32_e32 v0, 0, v0, vcc
+; CHECK-NEXT:    v_or_b32_e32 v0, v0, v1
+; CHECK-NEXT:    s_setpc_b64 s[30:31]
+  %sext = sext i8 %x to i32
+  %select = select i1 %cond, i32 %sext, i32 0
+  %or = or i32 %select, %y
+  ret i32 %or
+}

llvm/test/CodeGen/AMDGPU/sdwa-peephole-cndmask-wave64.mir

@@ -231,3 +231,41 @@ body:             |
     $vgpr0 = COPY %3
     SI_RETURN implicit $vgpr0
 ...
+
+# SDWA conversion of V_CNDMASK_B32 with V_BFE_I32 operand had to be
+# disabled.
+# FIXME sext modifier gets erroneously printed as neg modifier.
+
+...
+---
+name: issue138766_cndmask_with_sext
+tracksRegLiveness: true
+body:             |
+  bb.0:
+    liveins: $vgpr0, $vgpr1, $vgpr2
+
+    ; CHECK-LABEL: name: issue138766_cndmask_with_sext
+    ; CHECK: liveins: $vgpr0, $vgpr1, $vgpr2
+    ; CHECK-NEXT: {{  $}}
+    ; CHECK-NEXT: [[COPY:%[0-9]+]]:vgpr_32 = COPY $vgpr2
+    ; CHECK-NEXT: [[COPY1:%[0-9]+]]:vgpr_32 = COPY $vgpr1
+    ; CHECK-NEXT: [[COPY2:%[0-9]+]]:vgpr_32 = COPY $vgpr0
+    ; CHECK-NEXT: [[V_AND_B32_e64_:%[0-9]+]]:vgpr_32 = V_AND_B32_e64 1, [[COPY]], implicit $exec
+    ; CHECK-NEXT: [[V_CMP_EQ_U32_e64_:%[0-9]+]]:sreg_64_xexec = V_CMP_EQ_U32_e64 killed [[V_AND_B32_e64_]], 1, implicit $exec
+    ; CHECK-NEXT: [[V_BFE_I32_e64_:%[0-9]+]]:vgpr_32 = V_BFE_I32_e64 [[COPY2]], 0, 8, implicit $exec
+    ; CHECK-NEXT: $vcc = COPY killed [[V_CMP_EQ_U32_e64_]]
+    ; CHECK-NEXT: [[V_CNDMASK_B32_e32_:%[0-9]+]]:vgpr_32 = V_CNDMASK_B32_e32 0, killed [[V_BFE_I32_e64_]], implicit $vcc, implicit $exec
+    ; CHECK-NEXT: [[V_OR_B32_e64_:%[0-9]+]]:vgpr_32 = V_OR_B32_e64 killed [[V_CNDMASK_B32_e32_]], [[COPY1]], implicit $exec
+    ; CHECK-NEXT: $vgpr0 = COPY [[V_OR_B32_e64_]]
+    ; CHECK-NEXT: SI_RETURN implicit $vgpr0
+    %10:vgpr_32 = COPY $vgpr2
+    %9:vgpr_32 = COPY $vgpr1
+    %8:vgpr_32 = COPY $vgpr0
+    %11:vgpr_32 = V_AND_B32_e64 1, %10, implicit $exec
+    %12:sreg_64_xexec = V_CMP_EQ_U32_e64 killed %11, 1, implicit $exec
+    %14:vgpr_32 = V_BFE_I32_e64 %8, 0, 8, implicit $exec
+    %16:vgpr_32 = V_CNDMASK_B32_e64 0, 0, 0, killed %14, killed %12, implicit $exec
+    %18:vgpr_32 = V_OR_B32_e64 killed %16, %9, implicit $exec
+    $vgpr0 = COPY %18
+    SI_RETURN implicit $vgpr0
+...