Is there a preferred integer size?

[barbercolman]

Is there a preferred integer size? I have a program with a lot of 8 bit data. Is there an advantage to switching over to 32 bit integers? Specifically I want to know if there is penalty for 8bit verses 32 bit access? Recently I have heard the new ARM parts do not have a penalty for accessing 8bit data.

Thanks

[egoodii · Accepted solution]

While there may be no 'penalty for access' to 8/16-bit (as you can see below, where the PktCnts is 16 bytes and PktRSSI is 16 half-words), what Cortex M4 doesn't seem to have are any 8/16-bit MATH operations (ignoring the SIMD instructions...).  That is to say, for any address-operation/compare/math, the number needs to be a 32bit 'full native' quantity.  Often, values declared as a 'smaller size' incur the penalty of S/UXTB or S/UXTH instructions to extend to 32bits for any such usage.  You can see that the 'load' of bytes from the arrays for some math on them are 'directly extended' by the LDRB.W construct (and LDRH.W and STRH.W for half-words).  So, as you might see, 'it depends on what you are doing' whether there is a performance-hit for <32bits.  For counters, I do like the '(u)int_fast8_t' for portability, but for IAR at least that equates to uint32_t, as per these loop examples, where R4 is my counter 'i':

uint_fast8_t i;

                for(i=sizeof(PktCnts);i>0;i--)

??RFTestMain_39:

       0x8c08: 0x2010         MOVS      R0, #16                 ; 0x10

       0x8c0a: 0x0004         MOVS      R4, R0

                for(i=sizeof(PktCnts);i>0;i--)

??RFTestMain_45:

       0x8c0c: 0x2c00         CMP       R4, #0

       0x8c0e: 0xd017         BEQ.N     ??RFTestMain_46         ; 0x8c40

                    if( PktCnts[i-1] != 0 )

       0x8c10: 0x4897         LDR.N     R0, ??DataTable4_10     ; PktCnts

       0x8c12: 0x1820         ADDS      R0, R4, R0

       0x8c14: 0xf810 0x0c01  LDRB.W    R0, [R0, #-0x1]

       0x8c18: 0x2800         CMP       R0, #0

       0x8c1a: 0xd00f         BEQ.N     ??RFTestMain_47         ; 0x8c3c

                        PktRSSI[i-1] = PktRSSI[i-1]/PktCnts[i-1];

       0x8c1c: 0x4893         LDR.N     R0, ??DataTable4_9      ; PktRSSI

       0x8c1e: 0xeb10 0x0044  ADDS.W    R0, R0, R4, LSL #1

       0x8c22: 0xf830 0x0c02  LDRH.W    R0, [R0, #-0x2]

       0x8c26: 0x4992         LDR.N     R1, ??DataTable4_10     ; PktCnts

       0x8c28: 0x1861         ADDS      R1, R4, R1

       0x8c2a: 0xf811 0x1c01  LDRB.W    R1, [R1, #-0x1]

       0x8c2e: 0xfb90 0xf0f1  SDIV      R0, R0, R1

       0x8c32: 0x498e         LDR.N     R1, ??DataTable4_9      ; PktRSSI

       0x8c34: 0xeb11 0x0144  ADDS.W    R1, R1, R4, LSL #1

       0x8c38: 0xf821 0x0c02  STRH.W    R0, [R1, #-0x2]

                for(i=sizeof(PktCnts);i>0;i--)

??RFTestMain_47:

       0x8c3c: 0x1e64         SUBS      R4, R4, #1

       0x8c3e: 0xe7e5         B.N       ??RFTestMain_45         ; 0x8c0c

int32_t i;

                for(i=sizeof(PktCnts);i>0;i--)

??RFTestMain_39:

       0x8c08: 0x2010         MOVS      R0, #16                 ; 0x10

       0x8c0a: 0x0004         MOVS      R4, R0

                for(i=sizeof(PktCnts);i>0;i--)

??RFTestMain_45:

       0x8c0c: 0x2c01         CMP       R4, #1

       0x8c0e: 0xdb17         BLT.N     ??RFTestMain_46         ; 0x8c40

                    if( PktCnts[i-1] != 0 )

       0x8c10: 0x4897         LDR.N     R0, ??DataTable4_10     ; PktCnts

       0x8c12: 0x1820         ADDS      R0, R4, R0

       0x8c14: 0xf810 0x0c01  LDRB.W    R0, [R0, #-0x1]

       0x8c18: 0x2800         CMP       R0, #0

       0x8c1a: 0xd00f         BEQ.N     ??RFTestMain_47         ; 0x8c3c

                        PktRSSI[i-1] = PktRSSI[i-1]/PktCnts[i-1];

       0x8c1c: 0x4893         LDR.N     R0, ??DataTable4_9      ; PktRSSI

       0x8c1e: 0xeb10 0x0044  ADDS.W    R0, R0, R4, LSL #1

       0x8c22: 0xf830 0x0c02  LDRH.W    R0, [R0, #-0x2]

       0x8c26: 0x4992         LDR.N     R1, ??DataTable4_10     ; PktCnts

       0x8c28: 0x1861         ADDS      R1, R4, R1

       0x8c2a: 0xf811 0x1c01  LDRB.W    R1, [R1, #-0x1]

       0x8c2e: 0xfb90 0xf0f1  SDIV      R0, R0, R1

       0x8c32: 0x498e         LDR.N     R1, ??DataTable4_9      ; PktRSSI

       0x8c34: 0xeb11 0x0144  ADDS.W    R1, R1, R4, LSL #1

       0x8c38: 0xf821 0x0c02  STRH.W    R0, [R1, #-0x2]

                for(i=sizeof(PktCnts);i>0;i--)

??RFTestMain_47:

       0x8c3c: 0x1e64         SUBS      R4, R4, #1

       0x8c3e: 0xe7e5         B.N       ??RFTestMain_45         ; 0x8c0c

uint16_t i;

                for(i=sizeof(PktCnts);i>0;i--)

??RFTestMain_39:

       0x8c1c: 0x2010         MOVS      R0, #16                 ; 0x10

       0x8c1e: 0x0004         MOVS      R4, R0

                for(i=sizeof(PktCnts);i>0;i--)

??RFTestMain_45:

       0x8c20: 0xb2a4         UXTH      R4, R4

       0x8c22: 0x2c01         CMP       R4, #1

       0x8c24: 0xd31b         BCC.N     ??RFTestMain_46         ; 0x8c5e

                    if( PktCnts[i-1] != 0 )

       0x8c26: 0xb2a4         UXTH      R4, R4

       0x8c28: 0x48a4         LDR.N     R0, ??DataTable4_6      ; PktCnts

       0x8c2a: 0x1820         ADDS      R0, R4, R0

       0x8c2c: 0xf810 0x0c01  LDRB.W    R0, [R0, #-0x1]

       0x8c30: 0x2800         CMP       R0, #0

       0x8c32: 0xd012         BEQ.N     ??RFTestMain_47         ; 0x8c5a

                        PktRSSI[i-1] = PktRSSI[i-1]/PktCnts[i-1];

       0x8c34: 0xb2a4         UXTH      R4, R4

       0x8c36: 0x48a0         LDR.N     R0, ??DataTable4_5      ; PktRSSI

       0x8c38: 0xeb10 0x0044  ADDS.W    R0, R0, R4, LSL #1

       0x8c3c: 0xf830 0x0c02  LDRH.W    R0, [R0, #-0x2]

       0x8c40: 0xb2a4         UXTH      R4, R4

       0x8c42: 0x499e         LDR.N     R1, ??DataTable4_6      ; PktCnts

       0x8c44: 0x1861         ADDS      R1, R4, R1

       0x8c46: 0xf811 0x1c01  LDRB.W    R1, [R1, #-0x1]

       0x8c4a: 0xfb90 0xf0f1  SDIV      R0, R0, R1

       0x8c4e: 0xb2a4         UXTH      R4, R4

       0x8c50: 0x4999         LDR.N     R1, ??DataTable4_5      ; PktRSSI

       0x8c52: 0xeb11 0x0144  ADDS.W    R1, R1, R4, LSL #1

       0x8c56: 0xf821 0x0c02  STRH.W    R0, [R1, #-0x2]

                for(i=sizeof(PktCnts);i>0;i--)

??RFTestMain_47:

       0x8c5a: 0x1e64         SUBS      R4, R4, #1

       0x8c5c: 0xe7e0         B.N       ??RFTestMain_45         ; 0x8c20

int8_t i;

                for(i=sizeof(PktCnts);i>0;i--)

??RFTestMain_39:

       0x8c1c: 0x2010         MOVS      R0, #16                 ; 0x10

       0x8c1e: 0x0004         MOVS      R4, R0

                for(i=sizeof(PktCnts);i>0;i--)

??RFTestMain_45:

       0x8c20: 0xb264         SXTB      R4, R4

       0x8c22: 0x2c01         CMP       R4, #1

       0x8c24: 0xdb1b         BLT.N     ??RFTestMain_46         ; 0x8c5e

                    if( PktCnts[i-1] != 0 )

       0x8c26: 0xb264         SXTB      R4, R4

       0x8c28: 0x48a4         LDR.N     R0, ??DataTable4_6      ; PktCnts

       0x8c2a: 0x1820         ADDS      R0, R4, R0

       0x8c2c: 0xf810 0x0c01  LDRB.W    R0, [R0, #-0x1]

       0x8c30: 0x2800         CMP       R0, #0

       0x8c32: 0xd012         BEQ.N     ??RFTestMain_47         ; 0x8c5a

                        PktRSSI[i-1] = PktRSSI[i-1]/PktCnts[i-1];

       0x8c34: 0xb264         SXTB      R4, R4

       0x8c36: 0x48a0         LDR.N     R0, ??DataTable4_5      ; PktRSSI

       0x8c38: 0xeb10 0x0044  ADDS.W    R0, R0, R4, LSL #1

       0x8c3c: 0xf830 0x0c02  LDRH.W    R0, [R0, #-0x2]

       0x8c40: 0xb264         SXTB      R4, R4

       0x8c42: 0x499e         LDR.N     R1, ??DataTable4_6      ; PktCnts

       0x8c44: 0x1861         ADDS      R1, R4, R1

       0x8c46: 0xf811 0x1c01  LDRB.W    R1, [R1, #-0x1]

       0x8c4a: 0xfb90 0xf0f1  SDIV      R0, R0, R1

       0x8c4e: 0xb264         SXTB      R4, R4

       0x8c50: 0x4999         LDR.N     R1, ??DataTable4_5      ; PktRSSI

       0x8c52: 0xeb11 0x0144  ADDS.W    R1, R1, R4, LSL #1

       0x8c56: 0xf821 0x0c02  STRH.W    R0, [R1, #-0x2]

                for(i=sizeof(PktCnts);i>0;i--)

??RFTestMain_47:

       0x8c5a: 0x1e64         SUBS      R4, R4, #1

       0x8c5c: 0xe7e0         B.N       ??RFTestMain_45         ; 0x8c20

Of course the code-result looks 'much better' with some optimization turned on!  'Maximum speed' simply unrolls the loop, while 'minimum size' gets this loop result, where R0 is now 'i', and the array 'static addresses' are pre-loaded outside the loop in R7:

uint32_t i;

                for(i=sizeof(PktCnts);i>0;i--)

??RFTestMain_39:

       0x6834: 0x2010         MOVS      R0, #16                 ; 0x10

??RFTestMain_40:

       0x6836: 0x19c1         ADDS      R1, R0, R7

       0x6838: 0x7bc9         LDRB      R1, [R1, #0xf]

       0x683a: 0xb129         CBZ       R1, ??RFTestMain_41     ; 0x6848

                        PktRSSI[i-1] = PktRSSI[i-1]/PktCnts[i-1];

       0x683c: 0xeb07 0x0240  ADD.W     R2, R7, R0, LSL #1

       0x6840: 0x8c53         LDRH      R3, [R2, #0x22]

       0x6842: 0xfb93 0xf1f1  SDIV      R1, R3, R1

       0x6846: 0x8451         STRH      R1, [R2, #0x22]

                for(i=sizeof(PktCnts);i>0;i--)

??RFTestMain_41:

       0x6848: 0x1e40         SUBS      R0, R0, #1

                for(i=sizeof(PktCnts);i>0;i--)

       0x684a: 0xd1f4         BNE.N     ??RFTestMain_40         ; 0x6836

[bobpaddock]

In theory #include <stdint.h> then use (u)int_fast8_t as the type and your compiler will do the correct thing for the given architecture.

See: <stdint.h> documentation.

Alas sometime the theory fails.  Also depends how how much RAM you have and how much you can stand to waste using 32 bits to store 8.

Always trade offs.. :-(