diff --git a/litex/build/sim/core/Makefile b/litex/build/sim/core/Makefile
index 3737af82306abffc36fbcafa5b4f5072e8a7cb99..149789d8cb5bf16d96e270cdb9dffd015e30f7aa 100644
--- a/litex/build/sim/core/Makefile
+++ b/litex/build/sim/core/Makefile
@@ -1,7 +1,7 @@
 include variables.mak
 
 CC = gcc
-CFLAGS = -Wall -O3 -ggdb $(if $(COVERAGE), -DVM_COVERAGE)
+CFLAGS = -Wall -O0 -ggdb $(if $(COVERAGE), -DVM_COVERAGE)
 LDFLAGS = -lpthread -ljson-c -lm -lstdc++ -ldl -levent
 
 CC_SRCS ?= "--cc dut.v"