[core] add alternate cheat mode for fast zeroing

* Ctrl-Alt-Z can now be used to zero a drive, while skipping blocks that are detected empty
* Depending on your hardware, as well as the existing drive content, this strategy can greatly
  speed up zeroing operations, especially if the flash memory's read speed is much higher than
  its write speed.
* Closes #1174

src/format.c

@@ -62,7 +62,7 @@ static int task_number = 0;
 extern const int nb_steps[FS_MAX];
 extern uint32_t dur_mins, dur_secs;
 static int fs_index = 0, wintogo_index = -1;
-extern BOOL force_large_fat32, enable_ntfs_compression, lock_drive, zero_drive, enable_file_indexing, write_as_image;
+extern BOOL force_large_fat32, enable_ntfs_compression, lock_drive, zero_drive, fast_zeroing, enable_file_indexing, write_as_image;
 uint8_t *grub2_buf = NULL;
 long grub2_len;
 static BOOL WritePBR(HANDLE hLogicalDrive);
@@ -1548,11 +1548,12 @@ static BOOL WriteDrive(HANDLE hPhysicalDrive, HANDLE hSourceImage)
 {
 	BOOL s, ret = FALSE;
 	LARGE_INTEGER li;
-	DWORD rSize, wSize, BufSize;
+	DWORD rSize, wSize, xSize, BufSize;
 	uint64_t wb, target_size = hSourceImage?img_report.image_size:SelectedDrive.DiskSize;
 	int64_t bled_ret;
 	uint8_t *buffer = NULL;
-	int i;
+	uint8_t *cmp_buffer = NULL;
+	int i, throttle_fast_zeroing = 0;
 
 	// We poked the MBR and other stuff, so we need to rewind
 	li.QuadPart = 0;
@@ -1572,7 +1573,7 @@ static BOOL WriteDrive(HANDLE hPhysicalDrive, HANDLE hSourceImage)
 			goto out;
 		}
 	} else {
-		uprintf(hSourceImage?"Writing Image...":"Zeroing drive...");
+		uprintf(hSourceImage?"Writing Image...":fast_zeroing?"Fast-zeroing drive...":"Zeroing drive...");
 		// Our buffer size must be a multiple of the sector size and *ALIGNED* to the sector size
 		BufSize = ((DD_BUFFER_SIZE + SelectedDrive.SectorSize - 1) / SelectedDrive.SectorSize) * SelectedDrive.SectorSize;
 		buffer = (uint8_t*)_mm_malloc(BufSize, SelectedDrive.SectorSize);
@@ -1581,12 +1582,18 @@ static BOOL WriteDrive(HANDLE hPhysicalDrive, HANDLE hSourceImage)
 			uprintf("Could not allocate disk write buffer");
 			goto out;
 		}
-		// Sanity check
-		if ((uintptr_t)buffer % SelectedDrive.SectorSize != 0) {
-			FormatStatus = ERROR_SEVERITY_ERROR | FAC(FACILITY_STORAGE) | ERROR_READ_FAULT;
-			uprintf("Write buffer is not aligned");
+		assert((uintptr_t)buffer % SelectedDrive.SectorSize == 0);
+
+		// Clear buffer
+		memset(buffer, fast_zeroing ? 0xff : 0x00, BufSize);
+
+		cmp_buffer = (uint8_t*)_mm_malloc(BufSize, SelectedDrive.SectorSize);
+		if (cmp_buffer == NULL) {
+			FormatStatus = ERROR_SEVERITY_ERROR | FAC(FACILITY_STORAGE) | ERROR_NOT_ENOUGH_MEMORY;
+			uprintf("Could not allocate disk comparison buffer");
 			goto out;
 		}
+		assert((uintptr_t)cmp_buffer % SelectedDrive.SectorSize == 0);
 
 		// Don't bother trying for something clever, using double buffering overlapped and whatnot:
 		// With Windows' default optimizations, sync read + sync write for sequential operations
@@ -1596,7 +1603,7 @@ static BOOL WriteDrive(HANDLE hPhysicalDrive, HANDLE hSourceImage)
 			if (GetTickCount64() > LastRefresh + MAX_REFRESH) {
 				LastRefresh = GetTickCount64();
 				format_percent = (100.0f*wb) / (1.0f*target_size);
-				PrintInfo(0, hSourceImage?MSG_261:MSG_286, format_percent);
+				PrintInfo(0, hSourceImage?MSG_261:fast_zeroing?MSG_306:MSG_286, format_percent);
 				UpdateProgress(OP_FORMAT, format_percent);
 			}
 
@@ -1618,6 +1625,61 @@ static BOOL WriteDrive(HANDLE hPhysicalDrive, HANDLE hSourceImage)
 			// WriteFile fails unless the size is a multiple of sector size
 			if (rSize % SelectedDrive.SectorSize != 0)
 				rSize = ((rSize + SelectedDrive.SectorSize - 1) / SelectedDrive.SectorSize) * SelectedDrive.SectorSize;
+
+			// Fast-zeroing: Depending on your hardware, reading from flash may be much faster than writing, so
+			// we might speed things up by skipping empty blocks, or skipping the write if the data is the same.
+			// Notes: A block is declared empty when all bits are either 0 (zeros) or 1 (flash block erased).
+			// Also, a back-off strategy is used to limit reading.
+
+			if (throttle_fast_zeroing) {
+				throttle_fast_zeroing--;
+			} else if ((fast_zeroing) && (hSourceImage == NULL)) { // currently only enabled for erase
+
+				CHECK_FOR_USER_CANCEL;
+
+				// Read block and compare against the block that needs to be written
+				s = ReadFile(hPhysicalDrive, cmp_buffer, rSize, &xSize, NULL);
+				if ((!s) || (xSize != rSize) ) {
+					uprintf("Read error: Could not read data for comparison - %s", WindowsErrorString());
+					goto out;
+				}
+
+				// erase or write
+				if (hSourceImage == NULL) {
+					// Erase, check for an empty block
+					int *ptr = (int*)(cmp_buffer);
+					// Get first element
+					int value = ptr[0];
+					// Check all bits are the same
+					if ((value != 0) && (value != -1)) {
+						goto blocknotempty;
+					}
+					// Compare the rest of the block against the first element
+					for (i = 1; i < (int)(rSize/sizeof(int)); i++) {
+						if (ptr[i] != value)
+							goto blocknotempty;
+					}
+					// Block is empty, skip write
+					wSize = rSize;
+					continue;
+				blocknotempty:
+					;
+				} else if (memcmp(cmp_buffer, buffer, rSize) == 0) {
+					// Write, block is unchanged, skip write
+					wSize = rSize;
+					continue;
+				}
+
+				// Move the file pointer position back for writing
+				li.QuadPart = wb;
+				if (!SetFilePointerEx(hPhysicalDrive, li, NULL, FILE_BEGIN)) {
+					uprintf("Error: Could not reset position - %s", WindowsErrorString());
+					goto out;
+				}
+				// throttle read operations
+				throttle_fast_zeroing = 15;
+			}
+
 			for (i = 1; i <= WRITE_RETRIES; i++) {
 				CHECK_FOR_USER_CANCEL;
 				s = WriteFile(hPhysicalDrive, buffer, rSize, &wSize, NULL);
@@ -1649,6 +1711,7 @@ static BOOL WriteDrive(HANDLE hPhysicalDrive, HANDLE hSourceImage)
 	ret = TRUE;
 out:
 	safe_mm_free(buffer);
+	safe_mm_free(cmp_buffer);
 	return ret;
 }