diff --git a/lib/README b/lib/README
index 53835e1a..63fcf814 100644
--- a/lib/README
+++ b/lib/README
@@ -61,7 +61,8 @@ stm32f1.patch for the modifications.
 
 The stm32f4 directory contains code from STMicroelectronics:
   http://www.st.com/en/embedded-software/stm32cubef4.html
-version 1.24.0 (extracted 20190723).
+version 1.24.0 (extracted 20190723). It has been modified to compile
+with gcc's LTO feature. See stm32f4.patch for the modifications.
 
 The hub-ctrl directory contains code from:
   https://github.com/codazoda/hub-ctrl.c/
diff --git a/lib/stm32f4/stm32f4.patch b/lib/stm32f4/stm32f4.patch
new file mode 100644
index 00000000..4f5d88c3
--- /dev/null
+++ b/lib/stm32f4/stm32f4.patch
@@ -0,0 +1,13 @@
+diff --git a/lib/stm32f4/system_stm32f4xx.c b/lib/stm32f4/system_stm32f4xx.c
+index 11c18d1a..b94cfb2f 100644
+--- a/lib/stm32f4/system_stm32f4xx.c
++++ b/lib/stm32f4/system_stm32f4xx.c
+@@ -163,7 +163,7 @@ const uint8_t APBPrescTable[8]  = {0, 0, 0, 0, 1, 2, 3, 4};
+   * @param  None
+   * @retval None
+   */
+-void SystemInit(void)
++void __attribute__((externally_visible)) SystemInit(void)
+ {
+   /* FPU settings ------------------------------------------------------------*/
+   #if (__FPU_PRESENT == 1) && (__FPU_USED == 1)
diff --git a/lib/stm32f4/system_stm32f4xx.c b/lib/stm32f4/system_stm32f4xx.c
index 11c18d1a..b94cfb2f 100644
--- a/lib/stm32f4/system_stm32f4xx.c
+++ b/lib/stm32f4/system_stm32f4xx.c
@@ -163,7 +163,7 @@ const uint8_t APBPrescTable[8]  = {0, 0, 0, 0, 1, 2, 3, 4};
   * @param  None
   * @retval None
   */
-void SystemInit(void)
+void __attribute__((externally_visible)) SystemInit(void)
 {
   /* FPU settings ------------------------------------------------------------*/
   #if (__FPU_PRESENT == 1) && (__FPU_USED == 1)