roundss

Round Scalar Single-Precision

ROUNDSS xmm1, xmm2/m32, imm8

Rounds low float according to immediate mode.

Details

Rounds the low 32-bit single-precision floating-point value in xmm2/m32 according to the rounding mode specified in imm8 and stores the rounded result in the low 32 bits of xmm1, while preserving the upper 96 bits of xmm1. No CPU flags are affected. Rounding mode is selected via imm8[1:0]; bit 2 can suppress inexact exceptions.

Pseudocode Operation

mode ← imm8[1:0]; precision_exception_suppress ← imm8[2]; xmm1[0:31] ← round(src[0:31], mode, precision_exception_suppress); xmm1[32:127] ← xmm1[32:127];

Example

ROUNDSS xmm1, xmm2/m32, 3

Encoding

Binary Layout
66
+0
0F
+1
3A
+2
0A
+3
 
Format SSE4.1
Opcode 66 0F 3A 0A
Extension SSE4.1

Operands

  • dest
    128-bit XMM SIMD register
  • src1
    128-bit XMM SIMD register or Memory operand
  • src2
    8-bit signed immediate

Reference (Intel® SDM)

Instruction Forms

Opcode Instruction Op/En 64/32-bit Mode CPUID Description
66 0F 3A 0A /r ib ROUNDSS xmm1, xmm2/m32, imm8 RMI V/V SSE4_1 Round the low packed single precision floatingpoint value in xmm2/m32 and place the result in xmm1. The rounding mode is determined by imm8.
VEX.LIG.66.0F3A.WIG 0A /r ib VROUNDSS xmm1, xmm2, xmm3/m32, imm8 AVX RVMI V/V Round the low packed single precision floatingpoint value in xmm3/m32 and place the result in xmm1. The rounding mode is determined by imm8. Also, upper packed single precision floating-point values (bits[127:32]) from xmm2 are copied to xmm1[127:32].

Description

Round the single precision floating-point value in the lowest dword of the source operand (second operand) using the rounding mode specified in the immediate operand (third operand) and place the result in the destination operand (first operand). The rounding process rounds a single precision floating-point input to an integer value and returns the result as a single precision floating-point value in the lowest position. The upper three single precision floating-point values in the destination are retained. The immediate operand specifies control fields for the rounding operation, three bit fields are defined and shown in Figure 4-24. Bit 3 of the immediate byte controls processor behavior for a precision exception, bit 2 selects the source of rounding mode control. Bits 1:0 specify a non-sticky rounding-mode value (Table 4-21 lists the encoded values for rounding-mode field). The Precision Floating-Point Exception is signaled according to the immediate operand. If any source operand is an SNaN then it will be converted to a QNaN. If DAZ is set to ‘1 then denormals will be converted to zero before rounding. 128-bit Legacy SSE version: The first source operand and the destination operand are the same. Bits (MAXVL1:32) of the corresponding YMM destination register remain unchanged. VEX.128 encoded version: Bits (MAXVL-1:128) of the destination YMM register are zeroed.

Operation

IF (imm[2] = ‘1)
THEN     // rounding mode is determined by MXCSR.RC
DEST[31:0] := ConvertSPFPToInteger_M(SRC[31:0]);
ELSE     // rounding mode is determined by IMM8.RC
DEST[31:0] := ConvertSPFPToInteger_Imm(SRC[31:0]);
FI;
DEST[127:32] remains unchanged ;

ROUNDSS (128-bit Legacy SSE Version)
DEST[31:0] := RoundToInteger(SRC[31:0], ROUND_CONTROL)
DEST[MAXVL-1:32] (Unmodified)





ROUNDSS—Round Scalar Single Precision Floating-Point Values                                                                                Vol. 2B 4-590
VROUNDSS (VEX.128 Encoded Version)
DEST[31:0] := RoundToInteger(SRC2[31:0], ROUND_CONTROL)
DEST[127:32] := SRC1[127:32]
DEST[MAXVL-1:128] := 0

Intel C/C++ Compiler Intrinsic Equivalent

ROUNDSS __m128 mm_round_ss(__m128 dst, __m128 s1, int iRoundMode);
ROUNDSS __m128 mm_floor_ss(__m128 dst, __m128 s1);
ROUNDSS __m128 mm_ceil_ss(__m128 dst, __m128 s1);

Exceptions

SIMD Floating-Point Exceptions

Invalid (signaled only if SRC = SNaN). Precision (signaled only if imm[3] = ‘0; if imm[3] = ‘1, then the Precision Mask in the MXCSR is ignored and preci- sion exception is not signaled.) Note that Denormal is not signaled by ROUNDSS.

Other Exceptions

See Table 2-20, “Type 3 Class Exception Conditions.” ROUNDSS—Round Scalar Single Precision Floating-Point Values Vol. 2B 4-591