[AMDGPU] Fix bf16 inv2pi inline constant hadling (#82283)

Inline constant 1/(2*pi) has the truncated value 0x3e22. According to
the spec it is not rounded. A bf16 value in a nutshall is a fp32 value
with cleared 16 bites of mantissa. The value 0x3e22 converted to fp32 is
0.158203125 and the next representable value 0x3e23 means 0.1591796875.
The fp32 value of 1/(2*pi) = 0.15915494 cannot be represented in bf16.
Although since bf16 values are essentailly truncated fp32 values we can
use 0.15915494 as an idiomatic representation of 1/(2*pi) inline
constant. This is also consistent with sp3 behaviour. The patch fixes
the problem that value we are printing for inv2pi inline constant is not
parsed as inv2pi by the asm parser and gets rounded.

Author: rampitec

llvm/lib/Target/AMDGPU/AsmParser/AMDGPUAsmParser.cpp

@@ -2230,6 +2230,24 @@ void AMDGPUOperand::addLiteralImmOperand(MCInst &Inst, int64_t Val, bool ApplyMo
       // in predicate methods (isLiteralImm())
       llvm_unreachable("fp literal in 64-bit integer instruction.");
 
+    case AMDGPU::OPERAND_REG_IMM_BF16:
+    case AMDGPU::OPERAND_REG_IMM_BF16_DEFERRED:
+    case AMDGPU::OPERAND_REG_INLINE_C_BF16:
+    case AMDGPU::OPERAND_REG_INLINE_C_V2BF16:
+    case AMDGPU::OPERAND_REG_INLINE_AC_BF16:
+    case AMDGPU::OPERAND_REG_INLINE_AC_V2BF16:
+    case AMDGPU::OPERAND_REG_IMM_V2BF16:
+      if (AsmParser->hasInv2PiInlineImm() && Literal == 0x3fc45f306725feed) {
+        // This is the 1/(2*pi) which is going to be truncated to bf16 with the
+        // loss of precision. The constant represents ideomatic fp32 value of
+        // 1/(2*pi) = 0.15915494 since bf16 is in fact fp32 with cleared low 16
+        // bits. Prevent rounding below.
+        Inst.addOperand(MCOperand::createImm(0x3e22));
+        setImmKindLiteral();
+        return;
+      }
+      [[fallthrough]];
+
     case AMDGPU::OPERAND_REG_IMM_INT32:
     case AMDGPU::OPERAND_REG_IMM_FP32:
     case AMDGPU::OPERAND_REG_IMM_FP32_DEFERRED:
@@ -2238,24 +2256,17 @@ void AMDGPUOperand::addLiteralImmOperand(MCInst &Inst, int64_t Val, bool ApplyMo
     case AMDGPU::OPERAND_REG_INLINE_AC_INT32:
     case AMDGPU::OPERAND_REG_INLINE_AC_FP32:
     case AMDGPU::OPERAND_REG_IMM_INT16:
-    case AMDGPU::OPERAND_REG_IMM_BF16:
     case AMDGPU::OPERAND_REG_IMM_FP16:
-    case AMDGPU::OPERAND_REG_IMM_BF16_DEFERRED:
     case AMDGPU::OPERAND_REG_IMM_FP16_DEFERRED:
     case AMDGPU::OPERAND_REG_INLINE_C_INT16:
-    case AMDGPU::OPERAND_REG_INLINE_C_BF16:
     case AMDGPU::OPERAND_REG_INLINE_C_FP16:
     case AMDGPU::OPERAND_REG_INLINE_C_V2INT16:
-    case AMDGPU::OPERAND_REG_INLINE_C_V2BF16:
     case AMDGPU::OPERAND_REG_INLINE_C_V2FP16:
     case AMDGPU::OPERAND_REG_INLINE_AC_INT16:
-    case AMDGPU::OPERAND_REG_INLINE_AC_BF16:
     case AMDGPU::OPERAND_REG_INLINE_AC_FP16:
     case AMDGPU::OPERAND_REG_INLINE_AC_V2INT16:
-    case AMDGPU::OPERAND_REG_INLINE_AC_V2BF16:
     case AMDGPU::OPERAND_REG_INLINE_AC_V2FP16:
     case AMDGPU::OPERAND_REG_IMM_V2INT16:
-    case AMDGPU::OPERAND_REG_IMM_V2BF16:
     case AMDGPU::OPERAND_REG_IMM_V2FP16:
     case AMDGPU::OPERAND_REG_INLINE_C_V2FP32:
     case AMDGPU::OPERAND_REG_IMM_V2FP32:

llvm/test/MC/AMDGPU/bf16_imm.s

@@ -34,10 +34,17 @@ v_dot2_bf16_bf16 v2, v0, 4.0, v2
 v_dot2_bf16_bf16 v2, v0, -4.0, v2
 // CHECK: v_dot2_bf16_bf16 v2, v0, -4.0, v2       ; encoding: [0x02,0x00,0x67,0xd6,0x00,0xef,0x09,0x04]
 
-// FIXME: pi/2 rounded value is incorrect in the inst printer.
+// Check 1/(2*pi) rounded value and ideomatic fp32 0.15915494 value
+// which cannot be accurately represented in bf16.
 
 v_dot2_bf16_bf16 v2, v0, 0.158203125, v2
 // CHECK: v_dot2_bf16_bf16 v2, v0, 0.15915494, v2 ; encoding: [0x02,0x00,0x67,0xd6,0x00,0xf1,0x09,0x04]
 
+v_dot2_bf16_bf16 v2, v0, 0.15915494, v2
+// CHECK: v_dot2_bf16_bf16 v2, v0, 0.15915494, v2 ; encoding: [0x02,0x00,0x67,0xd6,0x00,0xf1,0x09,0x04]
+
 v_dot2_bf16_bf16 v2, v0, 0x3e22, v2
 // CHECK: v_dot2_bf16_bf16 v2, v0, 0.15915494, v2 ; encoding: [0x02,0x00,0x67,0xd6,0x00,0xf1,0x09,0x04]
+
+v_dot2_bf16_bf16 v2, v0, v2, 0.15915494
+// CHECK: v_dot2_bf16_bf16 v2, v0, v2, 0.15915494 ; encoding: [0x02,0x00,0x67,0xd6,0x00,0x05,0xe2,0x03]