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我使用FreeRTOS在GNU ARM工具鏈（gcc 5.4.1）上開發STM32F746（ARM Cortex-M7）。我正在使用以下CFLAGS集：gcc是否重新排序我的易失性變量？
-g -Werror -Wextra -O2 -fno-common -ffunction-sections -fmessage-length=0 -g -mcpu=cortex-m7 -mthumb -mfloat-abi=hard -mfpu=fpv5-sp-d16 -fno-common -ffunction-sections -fmessage-length=0 -g -mcpu=cortex-m7 -mthumb -mfloat-abi=hard -mfpu=fpv5-sp-d16
以下代碼在控制檯中顯示爲'0'，就像DMA控制器未通電一樣。通常情況下，在DMA控制器上寫入AHB1ENR寄存器的RCC_AHB1ENR_DMA1EN電源。
/* *** Other peripheral configurations *** */
RCC->AHB1ENR |= RCC_AHB1ENR_DMA1EN;
DMA1_Stream3->CR = DMA_SxCR_PL | DMA_SxCR_MINC |
DMA_SxCR_TCIE | DMA_SxCR_TEIE | DMA_SxCR_DMEIE;
DEBUG_PRINTF("%x\r\n", DMA1_Stream3->CR);
但下面的代碼打印DMA1_Stream3-> CR的良好值。
/* *** Other peripheral configurations *** */
RCC->AHB1ENR |= RCC_AHB1ENR_DMA1EN;
__asm__ __volatile__ ("" ::: "memory");
DMA1_Stream3->CR = DMA_SxCR_PL | DMA_SxCR_MINC |
DMA_SxCR_TCIE | DMA_SxCR_TEIE | DMA_SxCR_DMEIE;
DEBUG_PRINTF("%x\r\n", DMA1_Stream3->CR);
我從來沒有添加這些記憶障礙的任何其它控制器所以爲什麼我要爲這一個做到這一點？我正在考慮由GCC重新排序的加載/存儲指令要麼是計時問題。我檢查了CPU（STM32F746）或數據表的勘誤表，但沒有找到任何東西。
這裏是RCC結構的定義：
typedef struct
{
/* ... */
__IO uint32_t AHB1ENR;
/* ... */
} RCC_TypeDef;
這裏是DMA1_Stream3結構類型的定義：
typedef struct
{
__IO uint32_t CR; /*!< DMA stream x configuration register */
__IO uint32_t NDTR; /*!< DMA stream x number of data register */
__IO uint32_t PAR; /*!< DMA stream x peripheral address register */
__IO uint32_t M0AR; /*!< DMA stream x memory 0 address register */
__IO uint32_t M1AR; /*!< DMA stream x memory 1 address register */
__IO uint32_t FCR; /*!< DMA stream x FIFO control register */
} DMA_Stream_TypeDef;
__IO被定義爲揮發性的。
這裏是該函數生成的彙編代碼：
08003f4c <bsp_initSPIADIS>:
8003f4c: 4956 ldr r1, [pc, #344] ; (80040a8 <bsp_initSPIADIS+0x15c>)
8003f4e: 4a57 ldr r2, [pc, #348] ; (80040ac <bsp_initSPIADIS+0x160>)
8003f50: 6b08 ldr r0, [r1, #48] ; 0x30
8003f52: f440 7080 orr.w r0, r0, #256 ; 0x100
8003f56: b5f8 push {r3, r4, r5, r6, r7, lr}
8003f58: 6308 str r0, [r1, #48] ; 0x30
8003f5a: 2500 movs r5, #0
8003f5c: 6810 ldr r0, [r2, #0]
8003f5e: f240 3607 movw r6, #775 ; 0x307
8003f62: 4b53 ldr r3, [pc, #332] ; (80040b0 <bsp_initSPIADIS+0x164>)
8003f64: f44f 5eb8 mov.w lr, #5888 ; 0x1700
8003f68: f020 000c bic.w r0, r0, #12
8003f6c: 4c51 ldr r4, [pc, #324] ; (80040b4 <bsp_initSPIADIS+0x168>)
8003f6e: 4f52 ldr r7, [pc, #328] ; (80040b8 <bsp_initSPIADIS+0x16c>)
8003f70: 6010 str r0, [r2, #0]
8003f72: 6810 ldr r0, [r2, #0]
8003f74: f040 0008 orr.w r0, r0, #8
8003f78: 6010 str r0, [r2, #0]
8003f7a: 6890 ldr r0, [r2, #8]
8003f7c: f020 000c bic.w r0, r0, #12
8003f80: 6090 str r0, [r2, #8]
8003f82: 6890 ldr r0, [r2, #8]
8003f84: f040 0004 orr.w r0, r0, #4
8003f88: 6090 str r0, [r2, #8]
8003f8a: 6890 ldr r0, [r2, #8]
8003f8c: f040 0008 orr.w r0, r0, #8
8003f90: 6090 str r0, [r2, #8]
8003f92: 6a10 ldr r0, [r2, #32]
8003f94: f020 00f0 bic.w r0, r0, #240 ; 0xf0
8003f98: 6210 str r0, [r2, #32]
8003f9a: 6a10 ldr r0, [r2, #32]
8003f9c: f040 0010 orr.w r0, r0, #16
8003fa0: 6210 str r0, [r2, #32]
8003fa2: 6a10 ldr r0, [r2, #32]
8003fa4: f040 0040 orr.w r0, r0, #64 ; 0x40
8003fa8: 6210 str r0, [r2, #32]
8003faa: 6b08 ldr r0, [r1, #48] ; 0x30
8003fac: 4a43 ldr r2, [pc, #268] ; (80040bc <bsp_initSPIADIS+0x170>)
8003fae: f040 0002 orr.w r0, r0, #2
8003fb2: 6308 str r0, [r1, #48] ; 0x30
8003fb4: 6818 ldr r0, [r3, #0]
8003fb6: f020 5040 bic.w r0, r0, #805306368 ; 0x30000000
8003fba: 6018 str r0, [r3, #0]
8003fbc: 6818 ldr r0, [r3, #0]
8003fbe: f040 5000 orr.w r0, r0, #536870912 ; 0x20000000
8003fc2: 6018 str r0, [r3, #0]
8003fc4: 6898 ldr r0, [r3, #8]
8003fc6: f020 5040 bic.w r0, r0, #805306368 ; 0x30000000
8003fca: 6098 str r0, [r3, #8]
8003fcc: 6898 ldr r0, [r3, #8]
8003fce: f040 5080 orr.w r0, r0, #268435456 ; 0x10000000
8003fd2: 6098 str r0, [r3, #8]
8003fd4: 6898 ldr r0, [r3, #8]
8003fd6: f040 5000 orr.w r0, r0, #536870912 ; 0x20000000
8003fda: 6098 str r0, [r3, #8]
8003fdc: 6a58 ldr r0, [r3, #36] ; 0x24
8003fde: f020 6070 bic.w r0, r0, #251658240 ; 0xf000000
8003fe2: 6258 str r0, [r3, #36] ; 0x24
8003fe4: 6a58 ldr r0, [r3, #36] ; 0x24
8003fe6: f040 7080 orr.w r0, r0, #16777216 ; 0x1000000
8003fea: 6258 str r0, [r3, #36] ; 0x24
8003fec: 6a58 ldr r0, [r3, #36] ; 0x24
8003fee: f040 6080 orr.w r0, r0, #67108864 ; 0x4000000
8003ff2: 6258 str r0, [r3, #36] ; 0x24
8003ff4: 6b08 ldr r0, [r1, #48] ; 0x30
8003ff6: f040 0002 orr.w r0, r0, #2
8003ffa: 6308 str r0, [r1, #48] ; 0x30
8003ffc: 6818 ldr r0, [r3, #0]
8003ffe: f020 4040 bic.w r0, r0, #3221225472 ; 0xc0000000
8004002: 6018 str r0, [r3, #0]
8004004: 6818 ldr r0, [r3, #0]
8004006: f040 4000 orr.w r0, r0, #2147483648 ; 0x80000000
800400a: 6018 str r0, [r3, #0]
800400c: 6898 ldr r0, [r3, #8]
800400e: f020 4040 bic.w r0, r0, #3221225472 ; 0xc0000000
8004012: 6098 str r0, [r3, #8]
8004014: 6898 ldr r0, [r3, #8]
8004016: f040 4080 orr.w r0, r0, #1073741824 ; 0x40000000
800401a: 6098 str r0, [r3, #8]
800401c: 6898 ldr r0, [r3, #8]
800401e: f040 4000 orr.w r0, r0, #2147483648 ; 0x80000000
8004022: 6098 str r0, [r3, #8]
8004024: 6a58 ldr r0, [r3, #36] ; 0x24
8004026: f020 4070 bic.w r0, r0, #4026531840 ; 0xf0000000
800402a: 6258 str r0, [r3, #36] ; 0x24
800402c: 6a58 ldr r0, [r3, #36] ; 0x24
800402e: f040 5080 orr.w r0, r0, #268435456 ; 0x10000000
8004032: 6258 str r0, [r3, #36] ; 0x24
8004034: 6a58 ldr r0, [r3, #36] ; 0x24
8004036: f040 4080 orr.w r0, r0, #1073741824 ; 0x40000000
800403a: 6258 str r0, [r3, #36] ; 0x24
800403c: 6c0b ldr r3, [r1, #64] ; 0x40
800403e: 4820 ldr r0, [pc, #128] ; (80040c0 <bsp_initSPIADIS+0x174>)
8004040: f443 4380 orr.w r3, r3, #16384 ; 0x4000
8004044: 640b str r3, [r1, #64] ; 0x40
8004046: 61e5 str r5, [r4, #28]
8004048: 6026 str r6, [r4, #0]
800404a: 6823 ldr r3, [r4, #0]
800404c: 4e1d ldr r6, [pc, #116] ; (80040c4 <bsp_initSPIADIS+0x178>)
800404e: f023 0338 bic.w r3, r3, #56 ; 0x38
8004052: 6023 str r3, [r4, #0]
8004054: 6823 ldr r3, [r4, #0]
8004056: f043 0338 orr.w r3, r3, #56 ; 0x38
800405a: 6023 str r3, [r4, #0]
800405c: f8c4 e004 str.w lr, [r4, #4]
8004060: 6b0b ldr r3, [r1, #48] ; 0x30
8004062: 4c19 ldr r4, [pc, #100] ; (80040c8 <bsp_initSPIADIS+0x17c>)
8004064: f443 1300 orr.w r3, r3, #2097152 ; 0x200000
8004068: 630b str r3, [r1, #48] ; 0x30
800406a: 603a str r2, [r7, #0]
800406c: 6839 ldr r1, [r7, #0]
800406e: f000 faab bl 80045c8 <printf>
8004072: 4b16 ldr r3, [pc, #88] ; (80040cc <bsp_initSPIADIS+0x180>)
8004074: 4629 mov r1, r5
8004076: 2203 movs r2, #3
8004078: 601c str r4, [r3, #0]
800407a: 2001 movs r0, #1
800407c: f7fe f90e bl 800229c <xQueueGenericCreate>
8004080: 4629 mov r1, r5
8004082: 6170 str r0, [r6, #20]
8004084: 2203 movs r2, #3
8004086: 2001 movs r0, #1
8004088: f44f 4480 mov.w r4, #16384 ; 0x4000
800408c: f7fe f906 bl 800229c <xQueueGenericCreate>
8004090: 4b0f ldr r3, [pc, #60] ; (80040d0 <bsp_initSPIADIS+0x184>)
8004092: 2240 movs r2, #64 ; 0x40
8004094: f44f 4100 mov.w r1, #32768 ; 0x8000
8004098: 6130 str r0, [r6, #16]
800409a: 601c str r4, [r3, #0]
800409c: 6019 str r1, [r3, #0]
800409e: f883 230e strb.w r2, [r3, #782] ; 0x30e
80040a2: f883 230f strb.w r2, [r3, #783] ; 0x30f
80040a6: bdf8 pop {r3, r4, r5, r6, r7, pc}
80040a8: 40023800 andmi r3, r2, r0, lsl #16
80040ac: 40022000 andmi r2, r2, r0
80040b0: 40020400 andmi r0, r2, r0, lsl #8
80040b4: 40003800 andmi r3, r0, r0, lsl #16
80040b8: 40026058 andmi r6, r2, r8, asr r0
80040bc: 00030416 andeq r0, r3, r6, lsl r4
80040c0: 08008ec8 stmdaeq r0, {r3, r6, r7, r9, sl, fp, pc}
80040c4: 2000c2d0 ldrdcs ip, [r0], -r0
80040c8: 00030456 andeq r0, r3, r6, asr r4
80040cc: 40026070 andmi r6, r2, r0, ror r0
80040d0: e000e100 and lr, r0, r0, lsl #2
跨編譯域使用結構可能會導致失敗。我知道這是目前的潮流，但它會通過... –
順便說一句你的問題是什麼？ –
這可能是控制器啓動需要多長時間的時間限制。如果RCC_AHB1ENR_DMA1EN是256或0x100，那麼GCC是** NOT **在你的彙編程序中重新排序。我會建議在啓用和寫入之間添加一個延遲。控制器內存的地址可以幫助人們瞭解編譯器的彙編程序，除非您希望有人熟悉控制器以便在此處回答。即，「RCC」和「DMA1_Stream3」的物理地址。 –